Aggregator; Multi-Exit Discriminator - Enterasys Security Router X-PeditionTM User Manual

Overview

Aggregator

The AGGREGATOR attribute, as shown in
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.

Multi-Exit Discriminator

The MULTI_EXIT_DISC (MED) attribute, as shown in
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
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
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
command. This path is assigned a value of infinity.
6-8 Configuring the Border Gateway Protocol
Figure 6-5 Aggregate and Aggregator Attribute
command specifies the MED of redistributed routes that match a specified route
Figure 6-5, is added by the BGP speaker that formed
Figure 6-6, is applied to external (inter-AS)
bgp always-compare-med
bgp bestpath missing-as-worst
command.