H3C S5600 Series Operation Manual page 356

AS numbers are listed by the distances between the ASs and the local AS. The number of the AS that is
closest to the local AS is listed in the head, as shown in
Figure 5-6 AS_PATH attribute
D=8.0.0.0
(10)
AS 20
D=8.0.0.0
(20,10)
D=8.0.0.0
(30,20,10)
AS 30
Normally, a router with BGP employed discards the routes that contain local AS number in the
AS_PATH attribute. This eliminates routing loops.
You can use the peer allow-as-loop command to allow AS number repetition to meet some special
needs.
AS_PATH attribute can also be used to choose and filter routes. BGP chooses the routes containing
less AS numbers with shorter path under the same circumstances. For example, in
router in AS50 will choose the path passing through AS40 as the route to the router in AS 10.
In some applications, you can increase the number of AS numbers a BGP route contains through
routing policy to control BGP routing in a flexible way.
By configuring AS path filtering list, you can have BGP routes filtered by the AS numbers contained in
the AS-Path attribute.
3) NEXT_HOP
Different from that of the IGP, the NEXT_HOP attribute of a BGP route does not necessarily holds the
IP address of the neighbor router.
The NEXT_HOP attribute is set in the following ways.
When a BGP speaker advertises a route generated by itself to all its neighbors, it sets the
NEXT_HOP attribute of the routing information to the address of its own interface connecting to the
peer.
8.0.0.0
AS 10
D=8.0.0.0
(10)
AS 40
D=8.0.0.0
(40,10)
AS 50
5-6
Figure 5-6.
Figure 5-6, the BGP