H3C Technologies H3C S7500E Series Switches - Manual (Page 209)

6-6
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:
Figure 6-6 AS_PATH attribute
8.0.0.0
AS 10
D = 8.0.0.0
(10) D = 8.0.0.0
(10)
AS 20 AS 40
D = 8.0.0.0
(20,10)
AS 30 AS 50
D = 8.0.0.0
(30,20,10)
D = 8.0.0.0
(40,10)
In general, a BGP router does not receive routes containing the local AS number to avoid routing
loops.
The current implementation supports using the peer allow-as-loop command to receive routes
containing the local AS number to meet special requirements.
The AS_PATH attribute can be used for route selection and filtering. BGP gives priority to the route
with the shortest AS_PATH length if other factors are the same. As shown in the above figure, the BGP
router in AS50 gives priority to the route passing AS40 for sending data to the destination 8.0.0.0.
In some applications, you can apply a routing policy to control BGP route selection by modifying the
AS_PATH length.
By configuring an AS path filtering list, you can filter routes based on AS numbers contained in the
AS_PATH attribute.