Multiple Exit Discriminator (MED)

Topics:

set metric Command

The set metric command can be used in a route map to make paths more or less preferable:

The Multi Exit Discriminator (MED) is an optional attribute that can be used to influence path preference. It is non-transitive, meaning it is configured on a single appliance and not advertised to neighbors in update messages. In this section, we will consider the uses of the bgp always-compare-med and bgp deterministic-med commands.

bgp always-compare-med Command

The bgp always-compare-med command allows comparison of the MED values for paths from different ASs for path selection. A path with lower MED is preferred.

As an example, consider the following routes in the BGP table and the always-compare-med command is enabled:

Route2 would be the chosen path because it has the lowest MED.

If the always-compare-med command was disabled, MED would not be considered when comparing Route1 and Route2 because they have different AS paths. MED would be compared for only Route1 and Route3.

bgp deterministic-med Command

The selected route is also affected by the bgp deterministic-med command, which compares MED when choosing among routes advertised by different peers in the same autonomous system.

When the bgp deterministic-med command is enabled, routes from the same AS are grouped together, and the best routes of each group are compared. If the BGP table showed:

BGP would have a group of Route1 and a second group of Route2 and Route3 (the same AS).

The best of each group is compared. Route1 is the best of its group because it is the only route from AS 200.

Route1 is compared to the Route2, the best of group AS 400 (the lower MED).

Since the two routes are not from the same AS, the MED is not considered in the comparison. The external BGP route is preferred over the internal BGP route, making Route3 the best route.