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Use the AS_PATH attribute 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 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.
3.
NEXT_HOP
Different from IGP, the NEXT_HOP attribute may not be the IP address of a directly connected router. It
involves the following 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 configured, the NEXT_HOP attribute of the
equal-cost routes is modified. For load-balancing information, see “
Figure 77
NEXT_HOP attribute
4.
MED (MULTI_EXIT_DISC)
The MED attribute is exchanged between two neighboring ASs, each of which does not advertise the
attribute to any other AS.
Similar to metrics used by IGP, MED is used to determine the best route for traffic going into an AS. When
a BGP router obtains multiple routes to the same destination, but with different next hops, it considers the
route with the smallest MED value the best route given that other conditions are the same. As shown in
, traffic from AS10 to AS20 travels through Router B that is selected according to MED.
