5.1.4. MULTI_EXIT_DISC

5.1.4. MULTI_EXIT_DISC

The MULTI_EXIT_DISC is an optional non-transitive attribute that is intended to be used on external (inter-AS) links to discriminate among multiple exit or entry points to the same neighboring AS. The value of the MULTI_EXIT_DISC attribute is a four-octet unsigned number, called a metric. All other factors being equal, the exit point with the lower metric SHOULD be preferred. If received over EBGP, the MULTI_EXIT_DISC attribute MAY be propagated over IBGP to other BGP speakers within the same AS (see also 9.1.2.2). The MULTI_EXIT_DISC attribute received from a neighboring AS MUST NOT be propagated to other neighboring ASes.

A BGP speaker MUST implement a mechanism (based on local configuration) that allows the MULTI_EXIT_DISC attribute to be removed from a route. If a BGP speaker is configured to remove the MULTI_EXIT_DISC attribute from a route, then this removal MUST be done prior to determining the degree of preference of the route and prior to performing route selection (Decision Process phases 1 and 2).

An implementation MAY also (based on local configuration) alter the value of the MULTI_EXIT_DISC attribute received over EBGP. If a BGP speaker is configured to alter the value of the MULTI_EXIT_DISC attribute received over EBGP, then altering the value MUST be done prior to determining the degree of preference of the route and prior to performing route selection (Decision Process phases 1 and 2). See Section 9.1.2.2 for necessary restrictions on this.