Multi Exit Discriminator

• You can use the MED to influence path selection in neighbor autonomous systems.

• An AS can specify its preferred entry point using the MED in outgoing EBGP updates.

• The MED is not propagated outside of a receiving AS.

• The default value of the MED attribute is 0.

• The MED is called the "metric" in Cisco IOS software.

• A lower MED value means more preferred.

The MED attribute is a hint to external neighbors about the preferred path into an AS when multiple entry points exist. You can apply the MED attribute on outgoing updates to a neighboring AS to influence the route selection process in that AS. The MED attribute is useful only when there are multiple entry points into an AS.

The MED attribute, which is sent to an external neighbor, will be seen only within that AS. An AS that receives a route that contains the MED attribute will not advertise that MED beyond its local AS.

The default value of the MED attribute is 0. A lower value of the MED attribute indicates a more preferred path.

The MED attribute is considered a "weak" metric. In contrast with weight and local preference, a router will prefer a path with the smallest MED value but only if the weight, local preference, AS-path, and origin code attributes are equal. Using the MED may not yield the expected result if the neighboring AS modifies any of the stronger BGP route selection mechanisms.

Note The term that is used in Cisco IOS software for MED is "metric." The term "metric" also applies to the set command that is used in route-maps as well as all show and debug commands.

© 2005, Cisco Systems, Inc. Route Selection Using Attributes 4-61

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