H3C S5600 SERIES Operation Manual page 332

In fact, all the BGP route attributes can be classified into the following four categories.
Well-known mandatory attributes, which can be identified by any BGP routers. Route attributes of
this type are carried in Update messages. Without these attributes, routing information goes wrong.
Well-known discretionary attributes, which can be identified by any BGP routers. An Update
message can travel with or without this type of attributes.
Optional transitive attributes, which can be transmitted among ASs. Although attributes of this type
may not be supported by any BGP routers, routes with them can still be received and be forwarded
to BGP speakers.
Optional non-transitive attributes, which is dropped on the BGP routers that do not support them. In
this case, the attributes are not forwarded to other BGP routers.
Table 5-1 lists basic BGP route attributes and the categories they belong to.
Table 5-1 BGP route attributes and the corresponding categories
BGP route attribute
ORIGIN
AS_PATH
NEXT_HOP
LOCAL_PREF
ATOMIC_AGGREGATE
AGGREGATOR
COMMUNITY
MULTI_EXIT_DISC(MED)
ORIGINATOR_ID
CLUSTER_LIST
Primary route attributes
1) ORIGIN
The ORIGIN attribute holds the source of routing information. It indicates how a route becomes a BGP
route. The following describes the possible values of the ORIGIN attribute.
IGP: BGP routes with their ORIGIN attributes being IGP have the highest priority. They are added
to the BGP routing table through the network command.
EGP: BGP routes with their ORIGIN attributes being EGP are obtained through EGP.
Incomplete: BGP routes with their ORIGIN attributes being Incomplete have the least priority. This
value does not indicate that the BGP route is unreachable; it means the source of the BGP route
cannot be determined. The ORIGIN attribute of a BGP route imported through the import-route
command is Incomplete.
2) AS_PATH
The AS_PATH attribute holds the numbers of all the ASs that a route passes from the source to the
destination. AS numbers in this attribute are in the order the route passes the ASs. Before a BGP
speaker advertises a route to the BGP speakers of other ASs, it adds the local AS number to the head
of the AS number queue in the AS_PATH attribute. According to the AS_PATH attribute of a received
BGP route, a router can retrieve the information about the ASs the route passes. In AS_PATH attribute,
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
5-5
Category