
Customizing BGP Services
114065 Rev. A 6-5
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.
Table 6-1 lists 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
Table 6-2.
Table 6-1. 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), or
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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