H3C Technologies H3C S7500E Series Switches - Manual (Page 208)

6-5
BGP Path Attributes
Classification of path attributes
BGP path attributes are a group of parameters encapsulated in the path attributes field of update
messages. They give detailed route attributes information that can be used for route filtering and
selection.
Path attributes fall into four categories:
z Well-known mandatory: Must be recognized by all BGP routers and must be included in every
Update message. Routing information errors occur without this attribute.
z Well-known discretionary: Can be recognized by all BGP routers and optional to be included in
every Update message as needed.
z 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.
z 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 6-1 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
Usage of BGP path attributes
1) ORIGIN
ORIGIN is a well-known mandatory attribute, which defines the origin of routing information, that is,
how a route became a BGP route. It involves three types:
z IGP: Has the highest priority. Routes added to the BGP routing table using the network command
have the IGP attribute.
z EGP: Has the second highest priority. Routes obtained via EGP have the EGP attribute.