The multiexit discriminator (MED) attribute is non-transitive. It is defined as a metric that is used to convey the relative preference of entry points into an AS. The MED is assigned a value of zero by default when an update is generated. Lower MED values are the preferred values in an update. The MED is used to calculate the cost of using a particular router for reaching the next group in the AS.
Unlike the local preference attribute, the MED attribute is exchanged between ASs. However, the MED attribute is non-transitive; therefore, the MED value for a destination is reset to zero when it leaves the neighboring AS, unless the AS is explicitly configured to propagate MED information into other ASs.
A set of path exchanges by BGP routers (internal to an AS) can include several nearly identical paths to destination, with the MED value and the next-hop attributes being the only differentiators. The router should not aggregate, or sum, these paths. Instead, these paths should use the MED value to select the preferred—or best— exit point from the AS.
MED values received for the same destination from multiple ASs are usable only if the receiving router has the bgp always-compare-med configured. If the MED comparison is not configured, the MED value of the routing updates will not be used in the path selection process.
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