3Com Switch 4800G 24-Port Configuration Manual page 370

370 C 30: BGP C
HAPTER ONFIGURATION
n
determine ASs to route the massage back. The number of the AS closest to the
receiver's AS is leftmost, as shown below:
Figure 120 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.
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 121.
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