- Enterasys Security Router User's Guide

Overview
6-8 Configuring the Border Gateway Protocol
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-med command.
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-worst
command. This path is assigned a value of infinity.