H3C S7500E Series Ethernet Switches Operation Manual

Operation Manual – IPv4 Routing
H3C S7500E Series Ethernet Switches Chapter 5 BGP Configuration
5-7
Figure 5-6 AS_PATH attribute
In general, a BGP router does not receive routes containing the local AS number to
avoid routing loops.
 Note:
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 information 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.
3) NEXT_HOP
Different from IGP, the NEXT_HOP attribute of BGP may not be the IP address of a
neighboring router. It involves three types of values, as shown in
Figure 5-7.
z When advertising a self-originated route to an EBGP peer, a BGP speaker sets
the NEXT_HOP for the route to the address of its sending interface.