rfc4271
Network Working Group Y. Rekhter, Ed.
Request for Comments: 4271 T. Li, Ed.
Obsoletes: 1771 S. Hares, Ed.
Category: Standards Track January 2006
A Border Gateway Protocol 4 (BGP-4)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document discusses the Border Gateway Protocol (BGP), which is
an inter-Autonomous System routing protocol.
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network
reachability information includes information on the list of
Autonomous Systems (ASes) that reachability information traverses.
This information is sufficient for constructing a graph of AS
connectivity for this reachability from which routing loops may be
pruned, and, at the AS level, some policy decisions may be enforced.
BGP-4 provides a set of mechanisms for supporting Classless Inter-
Domain Routing (CIDR). These mechanisms include support for
advertising a set of destinations as an IP prefix, and eliminating
the concept of network "class" within BGP. BGP-4 also introduces
mechanisms that allow aggregation of routes, including aggregation of
AS paths.
This document obsoletes RFC 1771.
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Table of Contents
1. Introduction ....................................................4
1.1. Definition of Commonly Used Terms ..........................4
1.2. Specification of Requirements ..............................6
2. Acknowledgements ................................................6
3. Summary of Operation ............................................7
3.1. Routes: Advertisement and Storage ..........................9
3.2. Routing Information Base ..................................10
4. Message Formats ................................................11
4.1. Message Header Format .....................................12
4.2. OPEN Message Format .......................................13
4.3. UPDATE Message Format .....................................14
4.4. KEEPALIVE Message Format ..................................21
4.5. NOTIFICATION Message Format ...............................21
5. Path Attributes ................................................23
5.1. Path Attribute Usage ......................................25
5.1.1. ORIGIN .............................................25
5.1.2. AS_PATH ............................................25
5.1.3. NEXT_HOP ...........................................26
5.1.4. MULTI_EXIT_DISC ....................................28
5.1.5. LOCAL_PREF .........................................29
5.1.6. ATOMIC_AGGREGATE ...................................29
5.1.7. AGGREGATOR .........................................30
6. BGP Error Handling. ............................................30
6.1. Message Header Error Handling .............................31
6.2. OPEN Message Error Handling ...............................31
6.3. UPDATE Message Error Handling .............................32
6.4. NOTIFICATION Message Error Handling .......................34
6.5. Hold Timer Expired Error Handling .........................34
6.6. Finite State Machine Error Handling .......................35
6.7. Cease .....................................................35
6.8. BGP Connection Collision Detection ........................35
7. BGP Version Negotiation ........................................36
8. BGP Finite State Machine (FSM) .................................37
8.1. Events for the BGP FSM ....................................38
8.1.1. Optional Events Linked to Optional Session
Attributes .........................................38
8.1.2. Administrative Events ..............................42
8.1.3. Timer Events .......................................46
8.1.4. TCP Connection-Based Events ........................47
8.1.5. BGP Message-Based Events ...........................49
8.2. Description of FSM ........................................51
8.2.1. FSM Definition .....................................51
8.2.1.1. Terms "active" and "passive" ..............52
8.2.1.2. FSM and Collision Detection ...............52
8.2.1.3. FSM and Optional Session Attributes .......52
8.2.1.4. FSM Event Numbers .........................53
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8.2.1.5. FSM Actions that are Implementation
Dependent .................................53
8.2.2. Finite State Machine ...............................53
9. UPDATE Message Handling ........................................75
9.1. Decision Process ..........................................76
9.1.1. Phase 1: Calculation of Degree of Preference .......77
9.1.2. Phase 2: Route Selection ...........................77
9.1.2.1. Route Resolvability Condition .............79
9.1.2.2. Breaking Ties (Phase 2) ...................80
9.1.3. Phase 3: Route Dissemination .......................82
9.1.4. Overlapping Routes .................................83
9.2. Update-Send Process .......................................84
9.2.1. Controlling Routing Traffic Overhead ...............85
9.2.1.1. Frequency of Route Advertisement ..........85
9.2.1.2. Frequency of Route Origination ............85
9.2.2. Efficient Organization of Routing Information ......86
9.2.2.1. Information Reduction .....................86
9.2.2.2. Aggregating Routing Information ...........87
9.3. Route Selection Criteria ..................................89
9.4. Originating BGP routes ....................................89
10. BGP Timers ....................................................90
Appendix A. Comparison with RFC 1771 .............................92
Appendix B. Comparison with RFC 1267 .............................93
Appendix C. Comparison with RFC 1163 .............................93
Appendix D. Comparison with RFC 1105 .............................94
Appendix E. TCP Options that May Be Used with BGP ................94
Appendix F. Implementation Recommendations .......................95
Appendix F.1. Multiple Networks Per Message .........95
Appendix F.2. Reducing Route Flapping ...............96
Appendix F.3. Path Attribute Ordering ...............96
Appendix F.4. AS_SET Sorting ........................96
Appendix F.5. Control Over Version Negotiation ......96
Appendix F.6. Complex AS_PATH Aggregation ...........96
Security Considerations ...........................................97
IANA Considerations ...............................................99
Normative References .............................................101
Informative References ...........................................101
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1. Introduction
The Border Gateway Protocol (BGP) is an inter-Autonomous System
routing protocol.
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network
reachability information includes information on the list of
Autonomous Systems (ASes) that reachability information traverses.
This information is sufficient for constructing a graph of AS
connectivity for this reachability, from which routing loops may be
pruned and, at the AS level, some policy decisions may be enforced.
BGP-4 provides a set of mechanisms for supporting Classless Inter-
Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms include
support for advertising a set of destinations as an IP prefix and
eliminating the concept of network "class" within BGP. BGP-4 also
introduces mechanisms that allow aggregation of routes, including
aggregation of AS paths.
Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the IP
header of the packet. This, in turn, reflects the set of policy
decisions that can (and cannot) be enforced using BGP. BGP can
support only those policies conforming to the destination-based
forwarding paradigm.
1.1. Definition of Commonly Used Terms
This section provides definitions for terms that have a specific
meaning to the BGP protocol and that are used throughout the text.
Adj-RIB-In
The Adj-RIBs-In contains unprocessed routing information that has
been advertised to the local BGP speaker by its peers.
Adj-RIB-Out
The Adj-RIBs-Out contains the routes for advertisement to specific
peers by means of the local speaker's UPDATE messages.
Autonomous System (AS)
The classic definition of an Autonomous System is a set of routers
under a single technical administration, using an interior gateway
protocol (IGP) and common metrics to determine how to route
packets within the AS, and using an inter-AS routing protocol to
determine how to route packets to other ASes. Since this classic
definition was developed, it has become common for a single AS to
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use several IGPs and, sometimes, several sets of metrics within an
AS. The use of the term Autonomous System stresses the fact that,
even when multiple IGPs and metrics are used, the administration
of an AS appears to other ASes to have a single coherent interior
routing plan, and presents a consistent picture of the
destinations that are reachable through it.
BGP Identifier
A 4-octet unsigned integer that indicates the BGP Identifier of
the sender of BGP messages. A given BGP speaker sets the value of
its BGP Identifier to an IP address assigned to that BGP speaker.
The value of the BGP Identifier is determined upon startup and is
the same for every local interface and BGP peer.
BGP speaker
A router that implements BGP.
EBGP
External BGP (BGP connection between external peers).
External peer
Peer that is in a different Autonomous System than the local
system.
Feasible route
An advertised route that is available for use by the recipient.
IBGP
Internal BGP (BGP connection between internal peers).
Internal peer
Peer that is in the same Autonomous System as the local system.
IGP
Interior Gateway Protocol - a routing protocol used to exchange
routing information among routers within a single Autonomous
System.
Loc-RIB
The Loc-RIB contains the routes that have been selected by the
local BGP speaker's Decision Process.
NLRI
Network Layer Reachability Information.
Route
A unit of information that pairs a set of destinations with the
attributes of a path to those destinations. The set of
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destinations are systems whose IP addresses are contained in one
IP address prefix carried in the Network Layer Reachability
Information (NLRI) field of an UPDATE message. The path is the
information reported in the path attributes field of the same
UPDATE message.
RIB
Routing Information Base.
Unfeasible route
A previously advertised feasible route that is no longer available
for use.
1.2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Acknowledgements
This document was originally published as [RFC1267] in October 1991,
jointly authored by Kirk Lougheed and Yakov Rekhter.
We would like to express our thanks to Guy Almes, Len Bosack, and
Jeffrey C. Honig for their contributions to the earlier version
(BGP-1) of this document.
We would like to specially acknowledge numerous contributions by
Dennis Ferguson to the earlier version of this document.
We would like to explicitly thank Bob Braden for the review of the
earlier version (BGP-2) of this document, and for his constructive
and valuable comments.
We would also like to thank Bob Hinden, Director for Routing of the
Internet Engineering Steering Group, and the team of reviewers he
assembled to review the earlier version (BGP-2) of this document.
This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia
Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted
with a strong combination of toughness, professionalism, and
courtesy.
Certain sections of the document borrowed heavily from IDRP
[IS10747], which is the OSI counterpart of BGP. For this, credit
should be given to the ANSI X3S3.3 group chaired by Lyman Chapin and
to Charles Kunzinger, who was the IDRP editor within that group.
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We would also like to thank Benjamin Abarbanel, Enke Chen, Edward
Crabbe, Mike Craren, Vincent Gillet, Eric Gray, Jeffrey Haas, Dimitry
Haskin, Stephen Kent, John Krawczyk, David LeRoy, Dan Massey,
Jonathan Natale, Dan Pei, Mathew Richardson, John Scudder, John
Stewart III, Dave Thaler, Paul Traina, Russ White, Curtis Villamizar,
and Alex Zinin for their comments.
We would like to specially acknowledge Andrew Lange for his help in
preparing the final version of this document.
Finally, we would like to thank all the members of the IDR Working
Group for their ideas and the support they have given to this
document.
3. Summary of Operation
The Border Gateway Protocol (BGP) is an inter-Autonomous System
routing protocol. It is built on experience gained with EGP (as
defined in [RFC904]) and EGP usage in the NSFNET Backbone (as
described in [RFC1092] and [RFC1093]). For more BGP-related
information, see [RFC1772], [RFC1930], [RFC1997], and [RFC2858].
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network
reachability information includes information on the list of
Autonomous Systems (ASes) that reachability information traverses.
This information is sufficient for constructing a graph of AS
connectivity, from which routing loops may be pruned, and, at the AS
level, some policy decisions may be enforced.
In the context of this document, we assume that a BGP speaker
advertises to its peers only those routes that it uses itself (in
this context, a BGP speaker is said to "use" a BGP route if it is the
most preferred BGP route and is used in forwarding). All other cases
are outside the scope of this document.
In the context of this document, the term "IP address" refers to an
IP Version 4 address [RFC791].
Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the IP
header of the packet. This, in turn, reflects the set of policy
decisions that can (and cannot) be enforced using BGP. Note that
some policies cannot be supported by the destination-based forwarding
paradigm, and thus require techniques such as source routing (aka
explicit routing) to be enforced. Such policies cannot be enforced
using BGP either. For example, BGP does not enable one AS to send
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traffic to a neighboring AS for forwarding to some destination
(reachable through but) beyond that neighboring AS, intending that
the traffic take a different route to that taken by the traffic
originating in the neighboring AS (for that same destination). On
the other hand, BGP can support any policy conforming to the
destination-based forwarding paradigm.
BGP-4 provides a new set of mechanisms for supporting Classless
Inter-Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms
include support for advertising a set of destinations as an IP prefix
and eliminating the concept of a network "class" within BGP. BGP-4
also introduces mechanisms that allow aggregation of routes,
including aggregation of AS paths.
This document uses the term `Autonomous System' (AS) throughout. The
classic definition of an Autonomous System is a set of routers under
a single technical administration, using an interior gateway protocol
(IGP) and common metrics to determine how to route packets within the
AS, and using an inter-AS routing protocol to determine how to route
packets to other ASes. Since this classic definition was developed,
it has become common for a single AS to use several IGPs and,
sometimes, several sets of metrics within an AS. The use of the term
Autonomous System stresses the fact that, even when multiple IGPs and
metrics are used, the administration of an AS appears to other ASes
to have a single coherent interior routing plan and presents a
consistent picture of the destinations that are reachable through it.
BGP uses TCP [RFC793] as its transport protocol. This eliminates the
need to implement explicit update fragmentation, retransmission,
acknowledgement, and sequencing. BGP listens on TCP port 179. The
error notification mechanism used in BGP assumes that TCP supports a
"graceful" close (i.e., that all outstanding data will be delivered
before the connection is closed).
A TCP connection is formed between two systems. They exchange
messages to open and confirm the connection parameters.
The initial data flow is the portion of the BGP routing table that is
allowed by the export policy, called the Adj-Ribs-Out (see 3.2).
Incremental updates are sent as the routing tables change. BGP does
not require a periodic refresh of the routing table. To allow local
policy changes to have the correct effect without resetting any BGP
connections, a BGP speaker SHOULD either (a) retain the current
version of the routes advertised to it by all of its peers for the
duration of the connection, or (b) make use of the Route Refresh
extension [RFC2918].
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KEEPALIVE messages may be sent periodically to ensure that the
connection is live. NOTIFICATION messages are sent in response to
errors or special conditions. If a connection encounters an error
condition, a NOTIFICATION message is sent and the connection is
closed.
A peer in a different AS is referred to as an external peer, while a
peer in the same AS is referred to as an internal peer. Internal BGP
and external BGP are commonly abbreviated as IBGP and EBGP.
If a particular AS has multiple BGP speakers and is providing transit
service for other ASes, then care must be taken to ensure a
consistent view of routing within the AS. A consistent view of the
interior routes of the AS is provided by the IGP used within the AS.
For the purpose of this document, it is assumed that a consistent
view of the routes exterior to the AS is provided by having all BGP
speakers within the AS maintain IBGP with each other.
This document specifies the base behavior of the BGP protocol. This
behavior can be, and is, modified by extension specifications. When
the protocol is extended, the new behavior is fully documented in the
extension specifications.
3.1. Routes: Advertisement and Storage
For the purpose of this protocol, a route is defined as a unit of
information that pairs a set of destinations with the attributes of a
path to those destinations. The set of destinations are systems
whose IP addresses are contained in one IP address prefix that is
carried in the Network Layer Reachability Information (NLRI) field of
an UPDATE message, and the path is the information reported in the
path attributes field of the same UPDATE message.
Routes are advertised between BGP speakers in UPDATE messages.
Multiple routes that have the same path attributes can be advertised
in a single UPDATE message by including multiple prefixes in the NLRI
field of the UPDATE message.
Routes are stored in the Routing Information Bases (RIBs): namely,
the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out, as described in
Section 3.2.
If a BGP speaker chooses to advertise a previously received route, it
MAY add to, or modify, the path attributes of the route before
advertising it to a peer.
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BGP provides mechanisms by which a BGP speaker can inform its peers
that a previously advertised route is no longer available for use.
There are three methods by which a given BGP speaker can indicate
that a route has been withdrawn from service:
a) the IP prefix that expresses the destination for a previously
advertised route can be advertised in the WITHDRAWN ROUTES
field in the UPDATE message, thus marking the associated route
as being no longer available for use,
b) a replacement route with the same NLRI can be advertised, or
c) the BGP speaker connection can be closed, which implicitly
removes all routes the pair of speakers had advertised to each
other from service.
Changing the attribute(s) of a route is accomplished by advertising a
replacement route. The replacement route carries new (changed)
attributes and has the same address prefix as the original route.
3.2. Routing Information Base
The Routing Information Base (RIB) within a BGP speaker consists of
three distinct parts:
a) Adj-RIBs-In: The Adj-RIBs-In stores routing information learned
from inbound UPDATE messages that were received from other BGP
speakers. Their contents represent routes that are available
as input to the Decision Process.
b) Loc-RIB: The Loc-RIB contains the local routing information the
BGP speaker selected by applying its local policies to the
routing information contained in its Adj-RIBs-In. These are
the routes that will be used by the local BGP speaker. The
next hop for each of these routes MUST be resolvable via the
local BGP speaker's Routing Table.
c) Adj-RIBs-Out: The Adj-RIBs-Out stores information the local BGP
speaker selected for advertisement to its peers. The routing
information stored in the Adj-RIBs-Out will be carried in the
local BGP speaker's UPDATE messages and advertised to its
peers.
In summary, the Adj-RIBs-In contains unprocessed routing information
that has been advertised to the local BGP speaker by its peers; the
Loc-RIB contains the routes that have been selected by the local BGP
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speaker's Decision Process; and the Adj-RIBs-Out organizes the routes
for advertisement to specific peers (by means of the local speaker's
UPDATE messages).
Although the conceptual model distinguishes between Adj-RIBs-In,
Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an
implementation must maintain three separate copies of the routing
information. The choice of implementation (for example, 3 copies of
the information vs 1 copy with pointers) is not constrained by the
protocol.
Routing information that the BGP speaker uses to forward packets (or
to construct the forwarding table used for packet forwarding) is
maintained in the Routing Table. The Routing Table accumulates
routes to directly connected networks, static routes, routes learned
from the IGP protocols, and routes learned from BGP. Whether a
specific BGP route should be installed in the Routing Table, and
whether a BGP route should override a route to the same destination
installed by another source, is a local policy decision, and is not
specified in this document. In addition to actual packet forwarding,
the Routing Table is used for resolution of the next-hop addresses
specified in BGP updates (see Section 5.1.3).
4. Message Formats
This section describes message formats used by BGP.
BGP messages are sent over TCP connections. A message is processed
only after it is entirely received. The maximum message size is 4096
octets. All implementations are required to support this maximum
message size. The smallest message that may be sent consists of a
BGP header without a data portion (19 octets).
All multi-octet fields are in network byte order.
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4.1. Message Header Format
Each message has a fixed-size header. There may or may not be a data
portion following the header, depending on the message type. The
layout of these fields is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ +
| Marker |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Marker:
This 16-octet field is included for compatibility; it MUST be
set to all ones.
Length:
This 2-octet unsigned integer indicates the total length of the
message, including the header in octets. Thus, it allows one
to locate the (Marker field of the) next message in the TCP
stream. The value of the Length field MUST always be at least
19 and no greater than 4096, and MAY be further constrained,
depending on the message type. "padding" of extra data after
the message is not allowed. Therefore, the Length field MUST
have the smallest value required, given the rest of the
message.
Type:
This 1-octet unsigned integer indicates the type code of the
message. This document defines the following type codes:
1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE
[RFC2918] defines one more type code.
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4.2. OPEN Message Format
After a TCP connection is established, the first message sent by each
side is an OPEN message. If the OPEN message is acceptable, a
KEEPALIVE message confirming the OPEN is sent back.
In addition to the fixed-size BGP header, the OPEN message contains
the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Autonomous System |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt Parm Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Parameters (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version:
This 1-octet unsigned integer indicates the protocol version
number of the message. The current BGP version number is 4.
My Autonomous System:
This 2-octet unsigned integer indicates the Autonomous System
number of the sender.
Hold Time:
This 2-octet unsigned integer indicates the number of seconds
the sender proposes for the value of the Hold Timer. Upon
receipt of an OPEN message, a BGP speaker MUST calculate the
value of the Hold Timer by using the smaller of its configured
Hold Time and the Hold Time received in the OPEN message. The
Hold Time MUST be either zero or at least three seconds. An
implementation MAY reject connections on the basis of the Hold
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Time. The calculated value indicates the maximum number of
seconds that may elapse between the receipt of successive
KEEPALIVE and/or UPDATE messages from the sender.
BGP Identifier:
This 4-octet unsigned integer indicates the BGP Identifier of
the sender. A given BGP speaker sets the value of its BGP
Identifier to an IP address that is assigned to that BGP
speaker. The value of the BGP Identifier is determined upon
startup and is the same for every local interface and BGP peer.
Optional Parameters Length:
This 1-octet unsigned integer indicates the total length of the
Optional Parameters field in octets. If the value of this
field is zero, no Optional Parameters are present.
Optional Parameters:
This field contains a list of optional parameters, in which
each parameter is encoded as a <Parameter Type, Parameter
Length, Parameter Value> triplet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
| Parm. Type | Parm. Length | Parameter Value (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
Parameter Type is a one octet field that unambiguously
identifies individual parameters. Parameter Length is a one
octet field that contains the length of the Parameter Value
field in octets. Parameter Value is a variable length field
that is interpreted according to the value of the Parameter
Type field.
[RFC3392] defines the Capabilities Optional Parameter.
The minimum length of the OPEN message is 29 octets (including the
message header).
4.3. UPDATE Message Format
UPDATE messages are used to transfer routing information between BGP
peers. The information in the UPDATE message can be used to
construct a graph that describes the relationships of the various
Autonomous Systems. By applying rules to be discussed, routing
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information loops and some other anomalies may be detected and
removed from inter-AS routing.
An UPDATE message is used to advertise feasible routes that share
common path attributes to a peer, or to withdraw multiple unfeasible
routes from service (see 3.1). An UPDATE message MAY simultaneously
advertise a feasible route and withdraw multiple unfeasible routes
from service. The UPDATE message always includes the fixed-size BGP
header, and also includes the other fields, as shown below (note,
some of the shown fields may not be present in every UPDATE message):
+-----------------------------------------------------+
| Withdrawn Routes Length (2 octets) |
+-----------------------------------------------------+
| Withdrawn Routes (variable) |
+-----------------------------------------------------+
| Total Path Attribute Length (2 octets) |
+-----------------------------------------------------+
| Path Attributes (variable) |
+-----------------------------------------------------+
| Network Layer Reachability Information (variable) |
+-----------------------------------------------------+
Withdrawn Routes Length:
This 2-octets unsigned integer indicates the total length of
the Withdrawn Routes field in octets. Its value allows the
length of the Network Layer Reachability Information field to
be determined, as specified below.
A value of 0 indicates that no routes are being withdrawn from
service, and that the WITHDRAWN ROUTES field is not present in
this UPDATE message.
Withdrawn Routes:
This is a variable-length field that contains a list of IP
address prefixes for the routes that are being withdrawn from
service. Each IP address prefix is encoded as a 2-tuple of the
form <length, prefix>, whose fields are described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
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The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains an IP address prefix, followed by
the minimum number of trailing bits needed to make the end
of the field fall on an octet boundary. Note that the value
of trailing bits is irrelevant.
Total Path Attribute Length:
This 2-octet unsigned integer indicates the total length of the
Path Attributes field in octets. Its value allows the length
of the Network Layer Reachability field to be determined as
specified below.
A value of 0 indicates that neither the Network Layer
Reachability Information field nor the Path Attribute field is
present in this UPDATE message.
Path Attributes:
A variable-length sequence of path attributes is present in
every UPDATE message, except for an UPDATE message that carries
only the withdrawn routes. Each path attribute is a triple
<attribute type, attribute length, attribute value> of variable
length.
Attribute Type is a two-octet field that consists of the
Attribute Flags octet, followed by the Attribute Type Code
octet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The high-order bit (bit 0) of the Attribute Flags octet is the
Optional bit. It defines whether the attribute is optional (if
set to 1) or well-known (if set to 0).
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The second high-order bit (bit 1) of the Attribute Flags octet
is the Transitive bit. It defines whether an optional
attribute is transitive (if set to 1) or non-transitive (if set
to 0).
For well-known attributes, the Transitive bit MUST be set to 1.
(See Section 5 for a discussion of transitive attributes.)
The third high-order bit (bit 2) of the Attribute Flags octet
is the Partial bit. It defines whether the information
contained in the optional transitive attribute is partial (if
set to 1) or complete (if set to 0). For well-known attributes
and for optional non-transitive attributes, the Partial bit
MUST be set to 0.
The fourth high-order bit (bit 3) of the Attribute Flags octet
is the Extended Length bit. It defines whether the Attribute
Length is one octet (if set to 0) or two octets (if set to 1).
The lower-order four bits of the Attribute Flags octet are
unused. They MUST be zero when sent and MUST be ignored when
received.
The Attribute Type Code octet contains the Attribute Type Code.
Currently defined Attribute Type Codes are discussed in Section
5.
If the Extended Length bit of the Attribute Flags octet is set
to 0, the third octet of the Path Attribute contains the length
of the attribute data in octets.
If the Extended Length bit of the Attribute Flags octet is set
to 1, the third and fourth octets of the path attribute contain
the length of the attribute data in octets.
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The remaining octets of the Path Attribute represent the
attribute value and are interpreted according to the Attribute
Flags and the Attribute Type Code. The supported Attribute
Type Codes, and their attribute values and uses are as follows:
a) ORIGIN (Type Code 1):
ORIGIN is a well-known mandatory attribute that defines the
origin of the path information. The data octet can assume
the following values:
Value Meaning
0 IGP - Network Layer Reachability Information
is interior to the originating AS
1 EGP - Network Layer Reachability Information
learned via the EGP protocol [RFC904]
2 INCOMPLETE - Network Layer Reachability
Information learned by some other means
Usage of this attribute is defined in 5.1.1.
b) AS_PATH (Type Code 2):
AS_PATH is a well-known mandatory attribute that is composed
of a sequence of AS path segments. Each AS path segment is
represented by a triple <path segment type, path segment
length, path segment value>.
The path segment type is a 1-octet length field with the
following values defined:
Value Segment Type
1 AS_SET: unordered set of ASes a route in the
UPDATE message has traversed
2 AS_SEQUENCE: ordered set of ASes a route in
the UPDATE message has traversed
The path segment length is a 1-octet length field,
containing the number of ASes (not the number of octets) in
the path segment value field.
The path segment value field contains one or more AS
numbers, each encoded as a 2-octet length field.
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Usage of this attribute is defined in 5.1.2.
c) NEXT_HOP (Type Code 3):
This is a well-known mandatory attribute that defines the
(unicast) IP address of the router that SHOULD be used as
the next hop to the destinations listed in the Network Layer
Reachability Information field of the UPDATE message.
Usage of this attribute is defined in 5.1.3.
d) MULTI_EXIT_DISC (Type Code 4):
This is an optional non-transitive attribute that is a
four-octet unsigned integer. The value of this attribute
MAY be used by a BGP speaker's Decision Process to
discriminate among multiple entry points to a neighboring
autonomous system.
Usage of this attribute is defined in 5.1.4.
e) LOCAL_PREF (Type Code 5):
LOCAL_PREF is a well-known attribute that is a four-octet
unsigned integer. A BGP speaker uses it to inform its other
internal peers of the advertising speaker's degree of
preference for an advertised route.
Usage of this attribute is defined in 5.1.5.
f) ATOMIC_AGGREGATE (Type Code 6)
ATOMIC_AGGREGATE is a well-known discretionary attribute of
length 0.
Usage of this attribute is defined in 5.1.6.
g) AGGREGATOR (Type Code 7)
AGGREGATOR is an optional transitive attribute of length 6.
The attribute contains the last AS number that formed the
aggregate route (encoded as 2 octets), followed by the IP
address of the BGP speaker that formed the aggregate route
(encoded as 4 octets). This SHOULD be the same address as
the one used for the BGP Identifier of the speaker.
Usage of this attribute is defined in 5.1.7.
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Network Layer Reachability Information:
This variable length field contains a list of IP address
prefixes. The length, in octets, of the Network Layer
Reachability Information is not encoded explicitly, but can be
calculated as:
UPDATE message Length - 23 - Total Path Attributes Length
- Withdrawn Routes Length
where UPDATE message Length is the value encoded in the fixed-
size BGP header, Total Path Attribute Length, and Withdrawn
Routes Length are the values encoded in the variable part of
the UPDATE message, and 23 is a combined length of the fixed-
size BGP header, the Total Path Attribute Length field, and the
Withdrawn Routes Length field.
Reachability information is encoded as one or more 2-tuples of
the form <length, prefix>, whose fields are described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains an IP address prefix, followed by
enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of the trailing bits is
irrelevant.
The minimum length of the UPDATE message is 23 octets -- 19 octets
for the fixed header + 2 octets for the Withdrawn Routes Length + 2
octets for the Total Path Attribute Length (the value of Withdrawn
Routes Length is 0 and the value of Total Path Attribute Length is
0).
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An UPDATE message can advertise, at most, one set of path attributes,
but multiple destinations, provided that the destinations share these
attributes. All path attributes contained in a given UPDATE message
apply to all destinations carried in the NLRI field of the UPDATE
message.
An UPDATE message can list multiple routes that are to be withdrawn
from service. Each such route is identified by its destination
(expressed as an IP prefix), which unambiguously identifies the route
in the context of the BGP speaker - BGP speaker connection to which
it has been previously advertised.
An UPDATE message might advertise only routes that are to be
withdrawn from service, in which case the message will not include
path attributes or Network Layer Reachability Information.
Conversely, it may advertise only a feasible route, in which case the
WITHDRAWN ROUTES field need not be present.
An UPDATE message SHOULD NOT include the same address prefix in the
WITHDRAWN ROUTES and Network Layer Reachability Information fields.
However, a BGP speaker MUST be able to process UPDATE messages in
this form. A BGP speaker SHOULD treat an UPDATE message of this form
as though the WITHDRAWN ROUTES do not contain the address prefix.
4.4. KEEPALIVE Message Format
BGP does not use any TCP-based, keep-alive mechanism to determine if
peers are reachable. Instead, KEEPALIVE messages are exchanged
between peers often enough not to cause the Hold Timer to expire. A
reasonable maximum time between KEEPALIVE messages would be one third
of the Hold Time interval. KEEPALIVE messages MUST NOT be sent more
frequently than one per second. An implementation MAY adjust the
rate at which it sends KEEPALIVE messages as a function of the Hold
Time interval.
If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
messages MUST NOT be sent.
A KEEPALIVE message consists of only the message header and has a
length of 19 octets.
4.5. NOTIFICATION Message Format
A NOTIFICATION message is sent when an error condition is detected.
The BGP connection is closed immediately after it is sent.
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In addition to the fixed-size BGP header, the NOTIFICATION message
contains the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Error Code:
This 1-octet unsigned integer indicates the type of
NOTIFICATION. The following Error Codes have been defined:
Error Code Symbolic Name Reference
1 Message Header Error Section 6.1
2 OPEN Message Error Section 6.2
3 UPDATE Message Error Section 6.3
4 Hold Timer Expired Section 6.5
5 Finite State Machine Error Section 6.6
6 Cease Section 6.7
Error subcode:
This 1-octet unsigned integer provides more specific
information about the nature of the reported error. Each Error
Code may have one or more Error Subcodes associated with it.
If no appropriate Error Subcode is defined, then a zero
(Unspecific) value is used for the Error Subcode field.
Message Header Error subcodes:
1 - Connection Not Synchronized.
2 - Bad Message Length.
3 - Bad Message Type.
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OPEN Message Error subcodes:
1 - Unsupported Version Number.
2 - Bad Peer AS.
3 - Bad BGP Identifier.
4 - Unsupported Optional Parameter.
5 - [Deprecated - see Appendix A].
6 - Unacceptable Hold Time.
UPDATE Message Error subcodes:
1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute.
4 - Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid ORIGIN Attribute.
7 - [Deprecated - see Appendix A].
8 - Invalid NEXT_HOP Attribute.
9 - Optional Attribute Error.
10 - Invalid Network Field.
11 - Malformed AS_PATH.
Data:
This variable-length field is used to diagnose the reason for
the NOTIFICATION. The contents of the Data field depend upon
the Error Code and Error Subcode. See Section 6 for more
details.
Note that the length of the Data field can be determined from
the message Length field by the formula:
Message Length = 21 + Data Length
The minimum length of the NOTIFICATION message is 21 octets
(including message header).
5. Path Attributes
This section discusses the path attributes of the UPDATE message.
Path attributes fall into four separate categories:
1. Well-known mandatory.
2. Well-known discretionary.
3. Optional transitive.
4. Optional non-transitive.
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BGP implementations MUST recognize all well-known attributes. Some
of these attributes are mandatory and MUST be included in every
UPDATE message that contains NLRI. Others are discretionary and MAY
or MAY NOT be sent in a particular UPDATE message.
Once a BGP peer has updated any well-known attributes, it MUST pass
these attributes to its peers in any updates it transmits.
In addition to well-known attributes, each path MAY contain one or
more optional attributes. It is not required or expected that all
BGP implementations support all optional attributes. The handling of
an unrecognized optional attribute is determined by the setting of
the Transitive bit in the attribute flags octet. Paths with
unrecognized transitive optional attributes SHOULD be accepted. If a
path with an unrecognized transitive optional attribute is accepted
and passed to other BGP peers, then the unrecognized transitive
optional attribute of that path MUST be passed, along with the path,
to other BGP peers with the Partial bit in the Attribute Flags octet
set to 1. If a path with a recognized, transitive optional attribute
is accepted and passed along to other BGP peers and the Partial bit
in the Attribute Flags octet is set to 1 by some previous AS, it MUST
NOT be set back to 0 by the current AS. Unrecognized non-transitive
optional attributes MUST be quietly ignored and not passed along to
other BGP peers.
New, transitive optional attributes MAY be attached to the path by
the originator or by any other BGP speaker in the path. If they are
not attached by the originator, the Partial bit in the Attribute
Flags octet is set to 1. The rules for attaching new non-transitive
optional attributes will depend on the nature of the specific
attribute. The documentation of each new non-transitive optional
attribute will be expected to include such rules (the description of
the MULTI_EXIT_DISC attribute gives an example). All optional
attributes (both transitive and non-transitive), MAY be updated (if
appropriate) by BGP speakers in the path.
The sender of an UPDATE message SHOULD order path attributes within
the UPDATE message in ascending order of attribute type. The
receiver of an UPDATE message MUST be prepared to handle path
attributes within UPDATE messages that are out of order.
The same attribute (attribute with the same type) cannot appear more
than once within the Path Attributes field of a particular UPDATE
message.
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The mandatory category refers to an attribute that MUST be present in
both IBGP and EBGP exchanges if NLRI are contained in the UPDATE
message. Attributes classified as optional for the purpose of the
protocol extension mechanism may be purely discretionary,
discretionary, required, or disallowed in certain contexts.
attribute EBGP IBGP
ORIGIN mandatory mandatory
AS_PATH mandatory mandatory
NEXT_HOP mandatory mandatory
MULTI_EXIT_DISC discretionary discretionary
LOCAL_PREF see Section 5.1.5 required
ATOMIC_AGGREGATE see Section 5.1.6 and 9.1.4
AGGREGATOR discretionary discretionary
5.1. Path Attribute Usage
The usage of each BGP path attribute is described in the following
clauses.
5.1.1. ORIGIN
ORIGIN is a well-known mandatory attribute. The ORIGIN attribute is
generated by the speaker that originates the associated routing
information. Its value SHOULD NOT be changed by any other speaker.
5.1.2. AS_PATH
AS_PATH is a well-known mandatory attribute. This attribute
identifies the autonomous systems through which routing information
carried in this UPDATE message has passed. The components of this
list can be AS_SETs or AS_SEQUENCEs.
When a BGP speaker propagates a route it learned from another BGP
speaker's UPDATE message, it modifies the route's AS_PATH attribute
based on the location of the BGP speaker to which the route will be
sent:
a) When a given BGP speaker advertises the route to an internal
peer, the advertising speaker SHALL NOT modify the AS_PATH
attribute associated with the route.
b) When a given BGP speaker advertises the route to an external
peer, the advertising speaker updates the AS_PATH attribute as
follows:
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1) if the first path segment of the AS_PATH is of type
AS_SEQUENCE, the local system prepends its own AS number as
the last element of the sequence (put it in the leftmost
position with respect to the position of octets in the
protocol message). If the act of prepending will cause an
overflow in the AS_PATH segment (i.e., more than 255 ASes),
it SHOULD prepend a new segment of type AS_SEQUENCE and
prepend its own AS number to this new segment.
2) if the first path segment of the AS_PATH is of type AS_SET,
the local system prepends a new path segment of type
AS_SEQUENCE to the AS_PATH, including its own AS number in
that segment.
3) if the AS_PATH is empty, the local system creates a path
segment of type AS_SEQUENCE, places its own AS into that
segment, and places that segment into the AS_PATH.
When a BGP speaker originates a route then:
a) the originating speaker includes its own AS number in a path
segment, of type AS_SEQUENCE, in the AS_PATH attribute of all
UPDATE messages sent to an external peer. In this case, the AS
number of the originating speaker's autonomous system will be
the only entry the path segment, and this path segment will be
the only segment in the AS_PATH attribute.
b) the originating speaker includes an empty AS_PATH attribute in
all UPDATE messages sent to internal peers. (An empty AS_PATH
attribute is one whose length field contains the value zero).
Whenever the modification of the AS_PATH attribute calls for
including or prepending the AS number of the local system, the local
system MAY include/prepend more than one instance of its own AS
number in the AS_PATH attribute. This is controlled via local
configuration.
5.1.3. NEXT_HOP
The NEXT_HOP is a well-known mandatory attribute that defines the IP
address of the router that SHOULD be used as the next hop to the
destinations listed in the UPDATE message. The NEXT_HOP attribute is
calculated as follows:
1) When sending a message to an internal peer, if the route is not
locally originated, the BGP speaker SHOULD NOT modify the
NEXT_HOP attribute unless it has been explicitly configured to
announce its own IP address as the NEXT_HOP. When announcing a
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locally-originated route to an internal peer, the BGP speaker
SHOULD use the interface address of the router through which
the announced network is reachable for the speaker as the
NEXT_HOP. If the route is directly connected to the speaker,
or if the interface address of the router through which the
announced network is reachable for the speaker is the internal
peer's address, then the BGP speaker SHOULD use its own IP
address for the NEXT_HOP attribute (the address of the
interface that is used to reach the peer).
2) When sending a message to an external peer, X, and the peer is
one IP hop away from the speaker:
- If the route being announced was learned from an internal
peer or is locally originated, the BGP speaker can use an
interface address of the internal peer router (or the
internal router) through which the announced network is
reachable for the speaker for the NEXT_HOP attribute,
provided that peer X shares a common subnet with this
address. This is a form of "third party" NEXT_HOP attribute.
- Otherwise, if the route being announced was learned from an
external peer, the speaker can use an IP address of any
adjacent router (known from the received NEXT_HOP attribute)
that the speaker itself uses for local route calculation in
the NEXT_HOP attribute, provided that peer X shares a common
subnet with this address. This is a second form of "third
party" NEXT_HOP attribute.
- Otherwise, if the external peer to which the route is being
advertised shares a common subnet with one of the interfaces
of the announcing BGP speaker, the speaker MAY use the IP
address associated with such an interface in the NEXT_HOP
attribute. This is known as a "first party" NEXT_HOP
attribute.
- By default (if none of the above conditions apply), the BGP
speaker SHOULD use the IP address of the interface that the
speaker uses to establish the BGP connection to peer X in the
NEXT_HOP attribute.
3) When sending a message to an external peer X, and the peer is
multiple IP hops away from the speaker (aka "multihop EBGP"):
- The speaker MAY be configured to propagate the NEXT_HOP
attribute. In this case, when advertising a route that the
speaker learned from one of its peers, the NEXT_HOP attribute
of the advertised route is exactly the same as the NEXT_HOP
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attribute of the learned route (the speaker does not modify
the NEXT_HOP attribute).
- By default, the BGP speaker SHOULD use the IP address of the
interface that the speaker uses in the NEXT_HOP attribute to
establish the BGP connection to peer X.
Normally, the NEXT_HOP attribute is chosen such that the shortest
available path will be taken. A BGP speaker MUST be able to support
the disabling advertisement of third party NEXT_HOP attributes in
order to handle imperfectly bridged media.
A route originated by a BGP speaker SHALL NOT be advertised to a peer
using an address of that peer as NEXT_HOP. A BGP speaker SHALL NOT
install a route with itself as the next hop.
The NEXT_HOP attribute is used by the BGP speaker to determine the
actual outbound interface and immediate next-hop address that SHOULD
be used to forward transit packets to the associated destinations.
The immediate next-hop address is determined by performing a
recursive route lookup operation for the IP address in the NEXT_HOP
attribute, using the contents of the Routing Table, selecting one
entry if multiple entries of equal cost exist. The Routing Table
entry that resolves the IP address in the NEXT_HOP attribute will
always specify the outbound interface. If the entry specifies an
attached subnet, but does not specify a next-hop address, then the
address in the NEXT_HOP attribute SHOULD be used as the immediate
next-hop address. If the entry also specifies the next-hop address,
this address SHOULD be used as the immediate next-hop address for
packet forwarding.
5.1.4. MULTI_EXIT_DISC
The MULTI_EXIT_DISC is an optional non-transitive attribute that is
intended to be used on external (inter-AS) links to discriminate
among multiple exit or entry points to the same neighboring AS. The
value of the MULTI_EXIT_DISC attribute is a four-octet unsigned
number, called a metric. All other factors being equal, the exit
point with the lower metric SHOULD be preferred. If received over
EBGP, the MULTI_EXIT_DISC attribute MAY be propagated over IBGP to
other BGP speakers within the same AS (see also 9.1.2.2). The
MULTI_EXIT_DISC attribute received from a neighboring AS MUST NOT be
propagated to other neighboring ASes.
A BGP speaker MUST implement a mechanism (based on local
configuration) that allows the MULTI_EXIT_DISC attribute to be
removed from a route. If a BGP speaker is configured to remove the
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MULTI_EXIT_DISC attribute from a route, then this removal MUST be
done prior to determining the degree of preference of the route and
prior to performing route selection (Decision Process phases 1 and
2).
An implementation MAY also (based on local configuration) alter the
value of the MULTI_EXIT_DISC attribute received over EBGP. If a BGP
speaker is configured to alter the value of the MULTI_EXIT_DISC
attribute received over EBGP, then altering the value MUST be done
prior to determining the degree of preference of the route and prior
to performing route selection (Decision Process phases 1 and 2). See
Section 9.1.2.2 for necessary restrictions on this.
5.1.5. LOCAL_PREF
LOCAL_PREF is a well-known attribute that SHALL be included in all
UPDATE messages that a given BGP speaker sends to other internal
peers. A BGP speaker SHALL calculate the degree of preference for
each external route based on the locally-configured policy, and
include the degree of preference when advertising a route to its
internal peers. The higher degree of preference MUST be preferred.
A BGP speaker uses the degree of preference learned via LOCAL_PREF in
its Decision Process (see Section 9.1.1).
A BGP speaker MUST NOT include this attribute in UPDATE messages it
sends to external peers, except in the case of BGP Confederations
[RFC3065]. If it is contained in an UPDATE message that is received
from an external peer, then this attribute MUST be ignored by the
receiving speaker, except in the case of BGP Confederations
[RFC3065].
5.1.6. ATOMIC_AGGREGATE
ATOMIC_AGGREGATE is a well-known discretionary attribute.
When a BGP speaker aggregates several routes for the purpose of
advertisement to a particular peer, the AS_PATH of the aggregated
route normally includes an AS_SET formed from the set of ASes from
which the aggregate was formed. In many cases, the network
administrator can determine if the aggregate can safely be advertised
without the AS_SET, and without forming route loops.
If an aggregate excludes at least some of the AS numbers present in
the AS_PATH of the routes that are aggregated as a result of dropping
the AS_SET, the aggregated route, when advertised to the peer, SHOULD
include the ATOMIC_AGGREGATE attribute.
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A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute SHOULD NOT remove the attribute when propagating the route
to other speakers.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute MUST NOT make any NLRI of that route more specific (as
defined in 9.1.4) when advertising this route to other BGP speakers.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute needs to be aware of the fact that the actual path to
destinations, as specified in the NLRI of the route, while having the
loop-free property, may not be the path specified in the AS_PATH
attribute of the route.
5.1.7. AGGREGATOR
AGGREGATOR is an optional transitive attribute, which MAY be included
in updates that are formed by aggregation (see Section 9.2.2.2). A
BGP speaker that performs route aggregation MAY add the AGGREGATOR
attribute, which SHALL contain its own AS number and IP address. The
IP address SHOULD be the same as the BGP Identifier of the speaker.
6. BGP Error Handling.
This section describes actions to be taken when errors are detected
while processing BGP messages.
When any of the conditions described here are detected, a
NOTIFICATION message, with the indicated Error Code, Error Subcode,
and Data fields, is sent, and the BGP connection is closed (unless it
is explicitly stated that no NOTIFICATION message is to be sent and
the BGP connection is not to be closed). If no Error Subcode is
specified, then a zero MUST be used.
The phrase "the BGP connection is closed" means the TCP connection
has been closed, the associated Adj-RIB-In has been cleared, and all
resources for that BGP connection have been deallocated. Entries in
the Loc-RIB associated with the remote peer are marked as invalid.
The local system recalculates its best routes for the destinations of
the routes marked as invalid. Before the invalid routes are deleted
from the system, it advertises, to its peers, either withdraws for
the routes marked as invalid, or the new best routes before the
invalid routes are deleted from the system.
Unless specified explicitly, the Data field of the NOTIFICATION
message that is sent to indicate an error is empty.
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6.1. Message Header Error Handling
All errors detected while processing the Message Header MUST be
indicated by sending the NOTIFICATION message with the Error Code
Message Header Error. The Error Subcode elaborates on the specific
nature of the error.
The expected value of the Marker field of the message header is all
ones. If the Marker field of the message header is not as expected,
then a synchronization error has occurred and the Error Subcode MUST
be set to Connection Not Synchronized.
If at least one of the following is true:
- if the Length field of the message header is less than 19 or
greater than 4096, or
- if the Length field of an OPEN message is less than the minimum
length of the OPEN message, or
- if the Length field of an UPDATE message is less than the
minimum length of the UPDATE message, or
- if the Length field of a KEEPALIVE message is not equal to 19,
or
- if the Length field of a NOTIFICATION message is less than the
minimum length of the NOTIFICATION message,
then the Error Subcode MUST be set to Bad Message Length. The Data
field MUST contain the erroneous Length field.
If the Type field of the message header is not recognized, then the
Error Subcode MUST be set to Bad Message Type. The Data field MUST
contain the erroneous Type field.
6.2. OPEN Message Error Handling
All errors detected while processing the OPEN message MUST be
indicated by sending the NOTIFICATION message with the Error Code
OPEN Message Error. The Error Subcode elaborates on the specific
nature of the error.
If the version number in the Version field of the received OPEN
message is not supported, then the Error Subcode MUST be set to
Unsupported Version Number. The Data field is a 2-octet unsigned
integer, which indicates the largest, locally-supported version
number less than the version the remote BGP peer bid (as indicated in
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the received OPEN message), or if the smallest, locally-supported
version number is greater than the version the remote BGP peer bid,
then the smallest, locally-supported version number.
If the Autonomous System field of the OPEN message is unacceptable,
then the Error Subcode MUST be set to Bad Peer AS. The determination
of acceptable Autonomous System numbers is outside the scope of this
protocol.
If the Hold Time field of the OPEN message is unacceptable, then the
Error Subcode MUST be set to Unacceptable Hold Time. An
implementation MUST reject Hold Time values of one or two seconds.
An implementation MAY reject any proposed Hold Time. An
implementation that accepts a Hold Time MUST use the negotiated value
for the Hold Time.
If the BGP Identifier field of the OPEN message is syntactically
incorrect, then the Error Subcode MUST be set to Bad BGP Identifier.
Syntactic correctness means that the BGP Identifier field represents
a valid unicast IP host address.
If one of the Optional Parameters in the OPEN message is not
recognized, then the Error Subcode MUST be set to Unsupported
Optional Parameters.
If one of the Optional Parameters in the OPEN message is recognized,
but is malformed, then the Error Subcode MUST be set to 0
(Unspecific).
6.3. UPDATE Message Error Handling
All errors detected while processing the UPDATE message MUST be
indicated by sending the NOTIFICATION message with the Error Code
UPDATE Message Error. The error subcode elaborates on the specific
nature of the error.
Error checking of an UPDATE message begins by examining the path
attributes. If the Withdrawn Routes Length or Total Attribute Length
is too large (i.e., if Withdrawn Routes Length + Total Attribute
Length + 23 exceeds the message Length), then the Error Subcode MUST
be set to Malformed Attribute List.
If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode MUST be set to
Attribute Flags Error. The Data field MUST contain the erroneous
attribute (type, length, and value).
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If any recognized attribute has an Attribute Length that conflicts
with the expected length (based on the attribute type code), then the
Error Subcode MUST be set to Attribute Length Error. The Data field
MUST contain the erroneous attribute (type, length, and value).
If any of the well-known mandatory attributes are not present, then
the Error Subcode MUST be set to Missing Well-known Attribute. The
Data field MUST contain the Attribute Type Code of the missing,
well-known attribute.
If any of the well-known mandatory attributes are not recognized,
then the Error Subcode MUST be set to Unrecognized Well-known
Attribute. The Data field MUST contain the unrecognized attribute
(type, length, and value).
If the ORIGIN attribute has an undefined value, then the Error Sub-
code MUST be set to Invalid Origin Attribute. The Data field MUST
contain the unrecognized attribute (type, length, and value).
If the NEXT_HOP attribute field is syntactically incorrect, then the
Error Subcode MUST be set to Invalid NEXT_HOP Attribute. The Data
field MUST contain the incorrect attribute (type, length, and value).
Syntactic correctness means that the NEXT_HOP attribute represents a
valid IP host address.
The IP address in the NEXT_HOP MUST meet the following criteria to be
considered semantically correct:
a) It MUST NOT be the IP address of the receiving speaker.
b) In the case of an EBGP, where the sender and receiver are one
IP hop away from each other, either the IP address in the
NEXT_HOP MUST be the sender's IP address that is used to
establish the BGP connection, or the interface associated with
the NEXT_HOP IP address MUST share a common subnet with the
receiving BGP speaker.
If the NEXT_HOP attribute is semantically incorrect, the error SHOULD
be logged, and the route SHOULD be ignored. In this case, a
NOTIFICATION message SHOULD NOT be sent, and the connection SHOULD
NOT be closed.
The AS_PATH attribute is checked for syntactic correctness. If the
path is syntactically incorrect, then the Error Subcode MUST be set
to Malformed AS_PATH.
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If the UPDATE message is received from an external peer, the local
system MAY check whether the leftmost (with respect to the position
of octets in the protocol message) AS in the AS_PATH attribute is
equal to the autonomous system number of the peer that sent the
message. If the check determines this is not the case, the Error
Subcode MUST be set to Malformed AS_PATH.
If an optional attribute is recognized, then the value of this
attribute MUST be checked. If an error is detected, the attribute
MUST be discarded, and the Error Subcode MUST be set to Optional
Attribute Error. The Data field MUST contain the attribute (type,
length, and value).
If any attribute appears more than once in the UPDATE message, then
the Error Subcode MUST be set to Malformed Attribute List.
The NLRI field in the UPDATE message is checked for syntactic
validity. If the field is syntactically incorrect, then the Error
Subcode MUST be set to Invalid Network Field.
If a prefix in the NLRI field is semantically incorrect (e.g., an
unexpected multicast IP address), an error SHOULD be logged locally,
and the prefix SHOULD be ignored.
An UPDATE message that contains correct path attributes, but no NLRI,
SHALL be treated as a valid UPDATE message.
6.4. NOTIFICATION Message Error Handling
If a peer sends a NOTIFICATION message, and the receiver of the
message detects an error in that message, the receiver cannot use a
NOTIFICATION message to report this error back to the peer. Any such
error (e.g., an unrecognized Error Code or Error Subcode) SHOULD be
noticed, logged locally, and brought to the attention of the
administration of the peer. The means to do this, however, lies
outside the scope of this document.
6.5. Hold Timer Expired Error Handling
If a system does not receive successive KEEPALIVE, UPDATE, and/or
NOTIFICATION messages within the period specified in the Hold Time
field of the OPEN message, then the NOTIFICATION message with the
Hold Timer Expired Error Code is sent and the BGP connection is
closed.
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6.6. Finite State Machine Error Handling
Any error detected by the BGP Finite State Machine (e.g., receipt of
an unexpected event) is indicated by sending the NOTIFICATION message
with the Error Code Finite State Machine Error.
6.7. Cease
In the absence of any fatal errors (that are indicated in this
section), a BGP peer MAY choose, at any given time, to close its BGP
connection by sending the NOTIFICATION message with the Error Code
Cease. However, the Cease NOTIFICATION message MUST NOT be used when
a fatal error indicated by this section does exist.
A BGP speaker MAY support the ability to impose a locally-configured,
upper bound on the number of address prefixes the speaker is willing
to accept from a neighbor. When the upper bound is reached, the
speaker, under control of local configuration, either (a) discards
new address prefixes from the neighbor (while maintaining the BGP
connection with the neighbor), or (b) terminates the BGP connection
with the neighbor. If the BGP speaker decides to terminate its BGP
connection with a neighbor because the number of address prefixes
received from the neighbor exceeds the locally-configured, upper
bound, then the speaker MUST send the neighbor a NOTIFICATION message
with the Error Code Cease. The speaker MAY also log this locally.
6.8. BGP Connection Collision Detection
If a pair of BGP speakers try to establish a BGP connection with each
other simultaneously, then two parallel connections well be formed.
If the source IP address used by one of these connections is the same
as the destination IP address used by the other, and the destination
IP address used by the first connection is the same as the source IP
address used by the other, connection collision has occurred. In the
event of connection collision, one of the connections MUST be closed.
Based on the value of the BGP Identifier, a convention is established
for detecting which BGP connection is to be preserved when a
collision occurs. The convention is to compare the BGP Identifiers
of the peers involved in the collision and to retain only the
connection initiated by the BGP speaker with the higher-valued BGP
Identifier.
Upon receipt of an OPEN message, the local system MUST examine all of
its connections that are in the OpenConfirm state. A BGP speaker MAY
also examine connections in an OpenSent state if it knows the BGP
Identifier of the peer by means outside of the protocol. If, among
these connections, there is a connection to a remote BGP speaker
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whose BGP Identifier equals the one in the OPEN message, and this
connection collides with the connection over which the OPEN message
is received, then the local system performs the following collision
resolution procedure:
1) The BGP Identifier of the local system is compared to the BGP
Identifier of the remote system (as specified in the OPEN
message). Comparing BGP Identifiers is done by converting them
to host byte order and treating them as 4-octet unsigned
integers.
2) If the value of the local BGP Identifier is less than the
remote one, the local system closes the BGP connection that
already exists (the one that is already in the OpenConfirm
state), and accepts the BGP connection initiated by the remote
system.
3) Otherwise, the local system closes the newly created BGP
connection (the one associated with the newly received OPEN
message), and continues to use the existing one (the one that
is already in the OpenConfirm state).
Unless allowed via configuration, a connection collision with an
existing BGP connection that is in the Established state causes
closing of the newly created connection.
Note that a connection collision cannot be detected with connections
that are in Idle, Connect, or Active states.
Closing the BGP connection (that results from the collision
resolution procedure) is accomplished by sending the NOTIFICATION
message with the Error Code Cease.
7. BGP Version Negotiation
BGP speakers MAY negotiate the version of the protocol by making
multiple attempts at opening a BGP connection, starting with the
highest version number each BGP speaker supports. If an open attempt
fails with an Error Code, OPEN Message Error, and an Error Subcode,
Unsupported Version Number, then the BGP speaker has available the
version number it tried, the version number its peer tried, the
version number passed by its peer in the NOTIFICATION message, and
the version numbers it supports. If the two peers do support one or
more common versions, then this will allow them to rapidly determine
the highest common version. In order to support BGP version
negotiation, future versions of BGP MUST retain the format of the
OPEN and NOTIFICATION messages.
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8. BGP Finite State Machine (FSM)
The data structures and FSM described in this document are conceptual
and do not have to be implemented precisely as described here, as
long as the implementations support the described functionality and
they exhibit the same externally visible behavior.
This section specifies the BGP operation in terms of a Finite State
Machine (FSM). The section falls into two parts:
1) Description of Events for the State machine (Section 8.1)
2) Description of the FSM (Section 8.2)
Session attributes required (mandatory) for each connection are:
1) State
2) ConnectRetryCounter
3) ConnectRetryTimer
4) ConnectRetryTime
5) HoldTimer
6) HoldTime
7) KeepaliveTimer
8) KeepaliveTime
The state session attribute indicates the current state of the BGP
FSM. The ConnectRetryCounter indicates the number of times a BGP
peer has tried to establish a peer session.
The mandatory attributes related to timers are described in Section
10. Each timer has a "timer" and a "time" (the initial value).
The optional Session attributes are listed below. These optional
attributes may be supported, either per connection or per local
system:
1) AcceptConnectionsUnconfiguredPeers
2) AllowAutomaticStart
3) AllowAutomaticStop
4) CollisionDetectEstablishedState
5) DampPeerOscillations
6) DelayOpen
7) DelayOpenTime
8) DelayOpenTimer
9) IdleHoldTime
10) IdleHoldTimer
11) PassiveTcpEstablishment
12) SendNOTIFICATIONwithoutOPEN
13) TrackTcpState
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The optional session attributes support different features of the BGP
functionality that have implications for the BGP FSM state
transitions. Two groups of the attributes which relate to timers
are:
group 1: DelayOpen, DelayOpenTime, DelayOpenTimer
group 2: DampPeerOscillations, IdleHoldTime, IdleHoldTimer
The first parameter (DelayOpen, DampPeerOscillations) is an optional
attribute that indicates that the Timer function is active. The
"Time" value specifies the initial value for the "Timer"
(DelayOpenTime, IdleHoldTime). The "Timer" specifies the actual
timer.
Please refer to Section 8.1.1 for an explanation of the interaction
between these optional attributes and the events signaled to the
state machine. Section 8.2.1.3 also provides a short overview of the
different types of optional attributes (flags or timers).
8.1. Events for the BGP FSM
8.1.1. Optional Events Linked to Optional Session Attributes
The Inputs to the BGP FSM are events. Events can either be mandatory
or optional. Some optional events are linked to optional session
attributes. Optional session attributes enable several groups of FSM
functionality.
The linkage between FSM functionality, events, and the optional
session attributes are described below.
Group 1: Automatic Administrative Events (Start/Stop)
Optional Session Attributes: AllowAutomaticStart,
AllowAutomaticStop,
DampPeerOscillations,
IdleHoldTime, IdleHoldTimer
Option 1: AllowAutomaticStart
Description: A BGP peer connection can be started and stopped
by administrative control. This administrative
control can either be manual, based on operator
intervention, or under the control of logic that
is specific to a BGP implementation. The term
"automatic" refers to a start being issued to the
BGP peer connection FSM when such logic determines
that the BGP peer connection should be restarted.
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The AllowAutomaticStart attribute specifies that
this BGP connection supports automatic starting of
the BGP connection.
If the BGP implementation supports
AllowAutomaticStart, the peer may be repeatedly
restarted. Three other options control the rate
at which the automatic restart occurs:
DampPeerOscillations, IdleHoldTime, and the
IdleHoldTimer.
The DampPeerOscillations option specifies that the
implementation engages additional logic to damp
the oscillations of BGP peers in the face of
sequences of automatic start and automatic stop.
IdleHoldTime specifies the length of time the BGP
peer is held in the Idle state prior to allowing
the next automatic restart. The IdleHoldTimer is
the timer that holds the peer in Idle state.
An example of DampPeerOscillations logic is an
increase of the IdleHoldTime value if a BGP peer
oscillates connectivity (connected/disconnected)
repeatedly within a time period. To engage this
logic, a peer could connect and disconnect 10
times within 5 minutes. The IdleHoldTime value
would be reset from 0 to 120 seconds.
Values: TRUE or FALSE
Option 2: AllowAutomaticStop
Description: This BGP peer session optional attribute indicates
that the BGP connection allows "automatic"
stopping of the BGP connection. An "automatic"
stop is defined as a stop under the control of
implementation-specific logic. The
implementation-specific logic is outside the scope
of this specification.
Values: TRUE or FALSE
Option 3: DampPeerOscillations
Description: The DampPeerOscillations optional session
attribute indicates that the BGP connection is
using logic that damps BGP peer oscillations in
the Idle State.
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Value: TRUE or FALSE
Option 4: IdleHoldTime
Description: The IdleHoldTime is the value that is set in the
IdleHoldTimer.
Values: Time in seconds
Option 5: IdleHoldTimer
Description: The IdleHoldTimer aids in controlling BGP peer
oscillation. The IdleHoldTimer is used to keep
the BGP peer in Idle for a particular duration.
The IdleHoldTimer_Expires event is described in
Section 8.1.3.
Values: Time in seconds
Group 2: Unconfigured Peers
Optional Session Attributes: AcceptConnectionsUnconfiguredPeers
Option 1: AcceptConnectionsUnconfiguredPeers
Description: The BGP FSM optionally allows the acceptance of
BGP peer connections from neighbors that are not
pre-configured. The
"AcceptConnectionsUnconfiguredPeers" optional
session attribute allows the FSM to support the
state transitions that allow the implementation to
accept or reject these unconfigured peers.
The AcceptConnectionsUnconfiguredPeers has
security implications. Please refer to the BGP
Vulnerabilities document [RFC4272] for details.
Value: True or False
Group 3: TCP processing
Optional Session Attributes: PassiveTcpEstablishment,
TrackTcpState
Option 1: PassiveTcpEstablishment
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Description: This option indicates that the BGP FSM will
passively wait for the remote BGP peer to
establish the BGP TCP connection.
value: TRUE or FALSE
Option 2: TrackTcpState
Description: The BGP FSM normally tracks the end result of a
TCP connection attempt rather than individual TCP
messages. Optionally, the BGP FSM can support
additional interaction with the TCP connection
negotiation. The interaction with the TCP events
may increase the amount of logging the BGP peer
connection requires and the number of BGP FSM
changes.
Value: TRUE or FALSE
Group 4: BGP Message Processing
Optional Session Attributes: DelayOpen, DelayOpenTime,
DelayOpenTimer,
SendNOTIFICATIONwithoutOPEN,
CollisionDetectEstablishedState
Option 1: DelayOpen
Description: The DelayOpen optional session attribute allows
implementations to be configured to delay sending
an OPEN message for a specific time period
(DelayOpenTime). The delay allows the remote BGP
Peer time to send the first OPEN message.
Value: TRUE or FALSE
Option 2: DelayOpenTime
Description: The DelayOpenTime is the initial value set in the
DelayOpenTimer.
Value: Time in seconds
Option 3: DelayOpenTimer
Description: The DelayOpenTimer optional session attribute is
used to delay the sending of an OPEN message on a
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connection. The DelayOpenTimer_Expires event
(Event 12) is described in Section 8.1.3.
Value: Time in seconds
Option 4: SendNOTIFICATIONwithoutOPEN
Description: The SendNOTIFICATIONwithoutOPEN allows a peer to
send a NOTIFICATION without first sending an OPEN
message. Without this optional session attribute,
the BGP connection assumes that an OPEN message
must be sent by a peer prior to the peer sending a
NOTIFICATION message.
Value: True or False
Option 5: CollisionDetectEstablishedState
Description: Normally, a Detect Collision (see Section 6.8)
will be ignored in the Established state. This
optional session attribute indicates that this BGP
connection processes collisions in the Established
state.
Value: True or False
Note: The optional session attributes clarify the BGP FSM
description for existing features of BGP implementations.
The optional session attributes may be pre-defined for an
implementation and not readable via management interfaces
for existing correct implementations. As newer BGP MIBs
(version 2 and beyond) are supported, these fields will be
accessible via a management interface.
8.1.2. Administrative Events
An administrative event is an event in which the operator interface
and BGP Policy engine signal the BGP-finite state machine to start or
stop the BGP state machine. The basic start and stop indications are
augmented by optional connection attributes that signal a certain
type of start or stop mechanism to the BGP FSM. An example of this
combination is Event 5, AutomaticStart_with_PassiveTcpEstablishment.
With this event, the BGP implementation signals to the BGP FSM that
the implementation is using an Automatic Start with the option to use
a Passive TCP Establishment. The Passive TCP establishment signals
that this BGP FSM will wait for the remote side to start the TCP
establishment.
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Note that only Event 1 (ManualStart) and Event 2 (ManualStop) are
mandatory administrative events. All other administrative events are
optional (Events 3-8). Each event below has a name, definition,
status (mandatory or optional), and the optional session attributes
that SHOULD be set at each stage. When generating Event 1 through
Event 8 for the BGP FSM, the conditions specified in the "Optional
Attribute Status" section are verified. If any of these conditions
are not satisfied, then the local system should log an FSM error.
The settings of optional session attributes may be implicit in some
implementations, and therefore may not be set explicitly by an
external operator action. Section 8.2.1.5 describes these implicit
settings of the optional session attributes. The administrative
states described below may also be implicit in some implementations
and not directly configurable by an external operator.
Event 1: ManualStart
Definition: Local system administrator manually starts the peer
connection.
Status: Mandatory
Optional
Attribute
Status: The PassiveTcpEstablishment attribute SHOULD be set
to FALSE.
Event 2: ManualStop
Definition: Local system administrator manually stops the peer
connection.
Status: Mandatory
Optional
Attribute
Status: No interaction with any optional attributes.
Event 3: AutomaticStart
Definition: Local system automatically starts the BGP
connection.
Status: Optional, depending on local system
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Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD be set
to TRUE if this event occurs.
2) If the PassiveTcpEstablishment optional session
attribute is supported, it SHOULD be set to
FALSE.
3) If the DampPeerOscillations is supported, it
SHOULD be set to FALSE when this event occurs.
Event 4: ManualStart_with_PassiveTcpEstablishment
Definition: Local system administrator manually starts the peer
connection, but has PassiveTcpEstablishment
enabled. The PassiveTcpEstablishment optional
attribute indicates that the peer will listen prior
to establishing the connection.
Status: Optional, depending on local system
Optional
Attribute
Status: 1) The PassiveTcpEstablishment attribute SHOULD be
set to TRUE if this event occurs.
2) The DampPeerOscillations attribute SHOULD be set
to FALSE when this event occurs.
Event 5: AutomaticStart_with_PassiveTcpEstablishment
Definition: Local system automatically starts the BGP
connection with the PassiveTcpEstablishment
enabled. The PassiveTcpEstablishment optional
attribute indicates that the peer will listen prior
to establishing a connection.
Status: Optional, depending on local system
Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD be set
to TRUE.
2) The PassiveTcpEstablishment attribute SHOULD be
set to TRUE.
3) If the DampPeerOscillations attribute is
supported, the DampPeerOscillations SHOULD be
set to FALSE.
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Event 6: AutomaticStart_with_DampPeerOscillations
Definition: Local system automatically starts the BGP peer
connection with peer oscillation damping enabled.
The exact method of damping persistent peer
oscillations is determined by the implementation
and is outside the scope of this document.
Status: Optional, depending on local system.
Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD be set
to TRUE.
2) The DampPeerOscillations attribute SHOULD be set
to TRUE.
3) The PassiveTcpEstablishment attribute SHOULD be
set to FALSE.
Event 7: AutomaticStart_with_DampPeerOscillations_and_
PassiveTcpEstablishment
Definition: Local system automatically starts the BGP peer
connection with peer oscillation damping enabled
and PassiveTcpEstablishment enabled. The exact
method of damping persistent peer oscillations is
determined by the implementation and is outside the
scope of this document.
Status: Optional, depending on local system
Optional
Attributes
Status: 1) The AllowAutomaticStart attribute SHOULD be set
to TRUE.
2) The DampPeerOscillations attribute SHOULD be set
to TRUE.
3) The PassiveTcpEstablishment attribute SHOULD be
set to TRUE.
Event 8: AutomaticStop
Definition: Local system automatically stops the BGP
connection.
An example of an automatic stop event is exceeding
the number of prefixes for a given peer and the
local system automatically disconnecting the peer.
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Status: Optional, depending on local system
Optional
Attribute
Status: 1) The AllowAutomaticStop attribute SHOULD be TRUE.
8.1.3. Timer Events
Event 9: ConnectRetryTimer_Expires
Definition: An event generated when the ConnectRetryTimer
expires.
Status: Mandatory
Event 10: HoldTimer_Expires
Definition: An event generated when the HoldTimer expires.
Status: Mandatory
Event 11: KeepaliveTimer_Expires
Definition: An event generated when the KeepaliveTimer expires.
Status: Mandatory
Event 12: DelayOpenTimer_Expires
Definition: An event generated when the DelayOpenTimer expires.
Status: Optional
Optional
Attribute
Status: If this event occurs,
1) DelayOpen attribute SHOULD be set to TRUE,
2) DelayOpenTime attribute SHOULD be supported,
3) DelayOpenTimer SHOULD be supported.
Event 13: IdleHoldTimer_Expires
Definition: An event generated when the IdleHoldTimer expires,
indicating that the BGP connection has completed
waiting for the back-off period to prevent BGP peer
oscillation.
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The IdleHoldTimer is only used when the persistent
peer oscillation damping function is enabled by
setting the DampPeerOscillations optional attribute
to TRUE.
Implementations not implementing the persistent
peer oscillation damping function may not have the
IdleHoldTimer.
Status: Optional
Optional
Attribute
Status: If this event occurs:
1) DampPeerOscillations attribute SHOULD be set to
TRUE.
2) IdleHoldTimer SHOULD have just expired.
8.1.4. TCP Connection-Based Events
Event 14: TcpConnection_Valid
Definition: Event indicating the local system reception of a
TCP connection request with a valid source IP
address, TCP port, destination IP address, and TCP
Port. The definition of invalid source and invalid
destination IP address is determined by the
implementation.
BGP's destination port SHOULD be port 179, as
defined by IANA.
TCP connection request is denoted by the local
system receiving a TCP SYN.
Status: Optional
Optional
Attribute
Status: 1) The TrackTcpState attribute SHOULD be set to
TRUE if this event occurs.
Event 15: Tcp_CR_Invalid
Definition: Event indicating the local system reception of a
TCP connection request with either an invalid
source address or port number, or an invalid
destination address or port number.
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BGP destination port number SHOULD be 179, as
defined by IANA.
A TCP connection request occurs when the local
system receives a TCP SYN.
Status: Optional
Optional
Attribute
Status: 1) The TrackTcpState attribute should be set to
TRUE if this event occurs.
Event 16: Tcp_CR_Acked
Definition: Event indicating the local system's request to
establish a TCP connection to the remote peer.
The local system's TCP connection sent a TCP SYN,
received a TCP SYN/ACK message, and sent a TCP ACK.
Status: Mandatory
Event 17: TcpConnectionConfirmed
Definition: Event indicating that the local system has received
a confirmation that the TCP connection has been
established by the remote site.
The remote peer's TCP engine sent a TCP SYN. The
local peer's TCP engine sent a SYN, ACK message and
now has received a final ACK.
Status: Mandatory
Event 18: TcpConnectionFails
Definition: Event indicating that the local sys
