Configuring the MED attribute

BGP uses MED to determine the best route for traffic going into an AS. When a BGP router obtains from EBGP peers multiple routes to the same destination but with different next hops, it considers the route with the smallest MED value as the best route if other conditions are the same.

To configure the default MED value (IPv4):

To configure the default MED value (IPv6):

This task enables BGP to compare the MEDs of routes from different ASs.

To enable MED comparison for routes from different ASs (IPv4):

To enable MED comparison for routes from different ASs (IPv6):

This task enables BGP to compare the MEDs of routes from an AS.

As shown in Figure 55, Router D learns network 10.0.0.0 from both Router A and Router B. Because Router B has a smaller router ID, the route learned from Router B is optimal.

     Network            NextHop         MED        LocPrf     PrefVal Path/Ogn
 *>i  10.0.0.0          2.2.2.2         50                    0       300e
 * i                    3.3.3.3         50                    0       200e

When Router D learns network 10.0.0.0 from Router C, it compares the route with the optimal route in its routing table. Because Router C and Router B reside in different ASs, BGP does not compare the MEDs of the two routes. Router C has a smaller router ID than Router B so the route from Router C becomes optimal.

     Network            NextHop         MED        LocPrf     PrefVal Path/Ogn
 *>i  10.0.0.0          1.1.1.1         60                    0       200e
 * i  10.0.0.0          2.2.2.2         50                    0       300e
 * i                    3.3.3.3         50                    0       200e

However, Router C and Router A reside in the same AS, and Router C has a greater MED, so network 10.0.0.0 learned from Router C should not be optimal.

You can configure the bestroute compare-med command to enable MED comparison for routes from the same AS on Router D. After that, Router D puts the routes received from each AS into a group, selects the route with the lowest MED from each group, and compares routes from different groups. This mechanism avoids the above-mentioned problem. The following output shows the BGP routing table on Router D after this feature is enabled. Network 10.0.0.0 learned from Router B is the optimal route.

     Network            NextHop         MED        LocPrf     PrefVal Path/Ogn
 *>i  10.0.0.0          2.2.2.2         50                    0       300e
 * i                    3.3.3.3         50                    0       200e
 * i                    1.1.1.1         60                    0       200e

To enable MED comparison for routes on a per-AS basis (IPv4):

To enable MED comparison for routes on a per-AS basis (IPv6):

This task enables BGP to compare the MEDs of routes received from confederation peers. However, if a route received from a confederation peer has an AS number that does not belong to the confederation, BGP does not compare the route with other routes. For example, a confederation has three AS numbers 65006, 65007, and 65009. BGP receives three routes from different confederation peers. The AS_PATH attributes of these routes are 65006 65009, 65007 65009, and 65008 65009, and the MED values of them are 2, 3, and 1. Because the third route's AS_PATH attribute contains AS number 65008 that does not belong to the confederation, BGP does not compare it with other routes. As a result, the first route becomes the optimal route.

To enable MED comparison for routes from confederation peers (IPv4):

To enable MED comparison for routes from confederation peers (IPv6):