Avaya Configuring IP Exterior Gateway Protocols (BGP and EGP) Uživatelský manuál stáhnout pdf (Strana 31)

Exterior Gateway Protocols (BGP and EGP)

308628-15.0 Rev 00

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Path Attributes

A BGP-3 update message has a variable-length sequence of path attributes. Each

attribute includes an attribute value and an attribute description. Table 1-1

lists the

mandatory and optional BGP-3 path attributes.

A BGP-4 update message has a variable-length sequence of path attributes. Each

attribute includes an attribute value and an attribute description. Table 1-2

lists the

mandatory BGP-4 path attributes.

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 specifies 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 Mandatory Path Attributes

Attribute Description

AS path Contains a list of the ASs that must be traversed to reach the given

destinations

Origin Contains 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 an 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 Specifies the IP address of the router to use as a next hop for the

advertised destinations

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Avaya Configuring IP Exterior Gateway Protocols (BGP and EGP) Uživatelský manuál Strana 31