KB.15.15

Figure 57 NEXT_HOP attribute

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 if other conditions are the same. As

shown below, traffic from AS10 to AS20 travels through Router B that is selected according to

MED.

Figure 58 MED attribute

In general, BGP compares MEDs of routes received from the same AS only.

NOTE: The current implementation supports using the always-compare-med command to force

BGP to compare MED values of routes received from different ASs.

LOCAL_PREF

The LOCAL_PREF attribute is exchanged between iBGP peers only, and therefore is not advertised

to any other AS. It indicates the priority of a BGP router. LOCAL_PREF is used to determine the

best route for traffic leaving the local AS. When a BGP router obtains from several iBGP peers

multiple routes to the same destination but with different next hops, it considers the route with the

highest LOCAL_PREF value as the best route. As shown below, traffic from AS20 to AS10 travels

through Router C that is selected according to LOCAL_PREF.

BGP path attributes 305