
Configuring BGP Services
117356-A Rev. A 8-5
In Figure 8-2, AS 20 is the transit AS. It provides information about its internal
networks, as well as transit networks, to the remaining ASs. The IBGP
connections between BGP routers A, B, and C are necessary to provide consistent
information to the ASs.
BGP 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. T
able 8-1 describes the mandatory and optional BGP-3 path attributes.
The BGP-4 update message has the same format and contains the same 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 T
able 8-2.
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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