Enterasys Networks X-PeditionTM manual Aggregator, Multi-Exit Discriminator

Overview

Aggregator

The AGGREGATOR attribute, as shown in Figure 6-5, is added by the BGP speaker that formed the aggregate route. It includes the AS and router ID of the BGP speaker that originated the aggregate prefix. It is commonly used for debugging purposes.

Figure 6-5 Aggregate and Aggregator Attribute

Multi-Exit Discriminator

The MULTI_EXIT_DISC (MED) attribute, as shown in Figure 6-6, is applied to external (inter-AS) links to discriminate among multiple exit or entry points into the same neighboring AS (influencing ingress traffic). The MED informs external neighbors about the preferred path into an AS that has multiple entry points with the rule that a lower MED is the preferred path over a higher MED. Comparing MEDs is performed only among paths from the same AS by default but you can compare MEDs from different AS’s with the bgp always-compare-medcommand.

Unlike the local preference attribute, the MED is exchanged between AS’s, but a MED that enters an AS does not leave the AS. When an update enters the AS with a certain MED, that value is used for decision-making within the AS.

When BGP relays the routing update to another AS, the MED is reset to zero (unless the outgoing MED is set to a specific value). Unless otherwise specified, the XSR compares MEDs for paths from external neighbors occupying the same AS. MEDs from different AS’s typically are not comparable because the MED associated with a route usually indicates the AS internal topology.

The set metric command specifies the MED of redistributed routes that match a specified route map; be aware that routes even lacking a MED value are set as well. You can penalize a path without a MED as the least desirable path available using the bgp bestpath missing-as-worstcommand. This path is assigned a value of infinity.

6-8 Configuring the Border Gateway Protocol