KB.15.15

Figure 56 AS_PATH attribute

Usually a BGP router does not receive routes containing the local AS number to avoid routing

loops.

NOTE: The current implementation supports using the neighbor allow-as-loop command

to receive routes containing the local AS number.

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.

NEXT_HOP

Different from IGP, the NEXT_HOP attribute may not be the IP address of a directly connected

router. It involves three types of values, as shown in the following figure.

• 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 configured, the NEXT_HOP attribute

will be modified. For load-balancing information, refer to BGP Route Selection.

304 Border Gateway Protocol (BGP)