Avaya Configuring IP Exterior Gateway Protocols (BGP and EGP) Manual de usuario descargar pdf (Pagina 29)

Exterior Gateway Protocols (BGP/EGP)

308628-14.00 Rev 00

1-9

Path Attributes

A BGP-3 update message contains a variable-length sequence of path attributes,

each attribute consisting of an attribute value and an attribute description.

Table 1-1

describes the mandatory and optional BGP-3 path attributes.

The BGP-4 update message has the same format and mandatory attributes as the

BGP-3 update message, with the following additions:

• In place of the unreachable attribute that BGP-3 includes as part of the path

attribute description, the BGP-4 update includes an unreachable field. This

field specifies destinations that have become unreachable.

• In place of the BGP-3 optional attributes, a BGP-4 update message can

include the optional attributes described in Table 1-2

Table 1-1. BGP-3 Path Attributes

Attribute Description

AS path Mandatory attribute containing a list of the ASs that must be traversed

to reach the given destinations

Origin Mandatory attribute containing one of the following values:

• IGP (the path is valid all the way to the IGP of the originating AS)

• EGP (the path was advertised using EGP by the last AS in the AS

path)

• Incomplete (the path is valid only to the last AS in the AS path)

Next hop Mandatory attribute that defines the IP address of the router to use as a

next hop for the advertised destinations

Inter-AS Optional attribute used to choose between paths to the destinations

listed

Unreachable Discretionary attribute used to indicate destinations that have become

unreachable

Table 1-2. BGP-4 Optional Path Attributes

Attribute Description

Multiexit discriminator Chooses between paths to the destinations listed

Local preference Allows AS border routers to indicate the preference

they have assigned to a chosen route when

advertising it to IBGP peers

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Avaya Configuring IP Exterior Gateway Protocols (BGP and EGP) Manual de usuario Pagina 29