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100

101

Configuring IP Services

8-6

303528-A Rev 00

BGP Updates

BGP-3 and BGP-4 speakers exchange routing information in the form of routing

updates that include a network number and a list of autonomous systems that the

routing information has passed through (the AS path).

In addition, an update includes the following:

• List of path attributes

• Local preference value (BGP-4 only)

Path Attributes

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

Each attribute entry consists of an attribute value and a field describing the

attribute. Table 8-1

describes the mandatory and optional BGP-3 path attributes.

Table 8-1. 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

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