3Com MSR 50 Series Configuration Manual page 830

830 C 56: BGP C
HAPTER ONFIGURATION
n
determine ASs to route massages back. The number of the AS closest with the
receiver's AS is leftmost, as shown below:
Figure 236 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
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.
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 AS 50 gives priority to the route
passing AS 40 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
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.
When sending a received route to an EBGP peer, a BGP speaker sets the
■
NEXT_HOP for the route to the address of the sending interface.
When sending a route received from an EBGP peer to an IBGP peer, a BGP
■
speaker does not modify the NEXT_HOP attribute. If load-balancing is
8.0.0.0
AS 10
D=8.0.0.0
(10)
AS 40
D=8.0.0.0
(40,10)
AS 50
Figure 237.