3com S7906E Manuel De Montage
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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 1-1 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.
z
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: