0).
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An UPDATE message can list multiple routes that are to be withdrawn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withdrawn from service, in which case the message will not include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WITHDRAWN ROUTES field need not be present.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WITHDRAWN ROUTES and Network Layer Reachability Information fields.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as though the WITHDRAWN ROUTES do not contain the address prefix.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.
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.
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.
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.
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:
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
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
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
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.
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.