3Com Switch 8800 Advanced Software V5 Configuration Guide

BGP Overview 423
BGP Path Attributes Classification of path attributes
Path attributes fall into four categories:
■ Well-known mandatory: Must be recognized by all BGP routers and must be
included in every update message. Routing information error occurs without
this attribute.
■ Well-known discretionary: Can be recognized by all BGP routers and optional
to be included in every update message as needed.
■ Optional transitive: Transitive attribute between ASs. A BGP router not
supporting this attribute can still receive routes with this attribute and advertise
them to other peers.
■ Optional non-transitive: If a BGP router does not support this attribute, it will
not advertise routes with this attribute.
The usage of each BGP path attributes is described in the following table.
Usage of BGP path attributes
1 ORIGIN
ORIGIN is a well-known mandatory attribute and defines the origin of routing
information and how a route becomes a BGP route. It involves three types:
■ IGP: Has the highest priority. Routes added to the BGP routing table using the
network command have the IGP attribute.
■ EGP: Has the second highest priority. Routes obtained via EGP have the EGP
attribute.
■ incomplete: Has the lowest priority. The source of routes with this attribute is
unknown, which does not mean such routes are unavailable. The routes
redistributed from other routing protocols have the incomplete attribute.
2 AS_PATH
AS_PATH is a well-known mandatory attribute. This attribute identifies the
autonomous systems through which routing information carried in this Update
message has passed. When a route is advertised from the local AS to another AS,
each passed AS number is added into the AS_PATH attribute, thus the receiver can
Tab le 23 Usage of BGP path attributes
Name Category
ORIGIN Well-known mandatory
AS_PATH Well-known mandatory
NEXT_HOP Well-known mandatory
LOCAL_PREF Well-known discretionary
ATOMIC_AGGREGATE Well-known discretionary
AGGREGATOR Optional transitive
COMMUNITY Optional transitive
MULTI_EXIT_DISC (MED) Optional non-transitive
ORIGINATOR_ID Optional non-transitive
CLUSTER_LIST Optional non-transitive