3Com Switch 4800G 24-Port Configuration Manual page 369

BGP Path Attributes
1 ORIGIN
2 AS_PATH
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 attribute is described in the following table.
Table 46 Usage of BGP path attributes
Name
ORIGIN
AS_PATH
NEXT_HOP
LOCAL_PREF
ATOMIC_AGGREGATE
AGGREGATOR
COMMUNITY
MULTI_EXIT_DISC (MED)
ORIGINATOR_ID
CLUSTER_LIST
Usage of BGP path attributes
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
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network command have the IGP attribute.
EGP: Has the second highest priority. Routes obtained via EGP have the EGP
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attribute.
incomplete: Has the lowest priority. The source of routes with this attribute is
■
unknown, which does not mean such routes are unreachable. The routes
redistributed from other routing protocols have the incomplete attribute.
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
BGP Overview
Category
Well-known mandatory
Well-known mandatory
Well-known mandatory
Well-known discretionary
Well-known discretionary
Optional transitive
Optional transitive
Optional non-transitive
Optional non-transitive
Optional non-transitive
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