H3C Technologies H3C SecPath F1000-E - Manual (Page 331)

5
Name Category
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:
• 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 unreachable. 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 determine ASs to route the massage back. The number of the AS closest to the receiver’s AS
is leftmost, as shown below:
Figure 6 AS_PATH attribute
In general, a BGP router does not receive routes containing the local AS number to avoid routing loops.