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Configuring IP Services
8-12
117356-B 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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