View - ADTRAN Support Community

Additional BGP Configuration
The algorithm BGP uses to select routes relies on many factors, some of which are
dependent upon configurations on the remote router. It is impossible to ensure that the
route with the lower metric will actually be selected.
This metric is sometimes called the multi-exit discriminator or MED because it is used to differentiate
routes sent over various external connections to the same neighboring AS.
When MEDs are used, routes to a specific part of the network are typically classified according to their
destination address. This classification is accomplished using one prefix list or several; depending on
the network setup and the goal. Refer to Prefix List on page 21 for information on how to create prefix
lists. Separate route maps are then configured for each neighbor to which the router connects. Refer to
Route Map on page 23 for information on how to create a route map. A prefix list is associated with
each route map entry. Again, depending on the network setup and the goal, the same prefix list can be
associated with each route map entry or there might be a different prefix list associated with every
route map entry.
Within each route map entry, use the set metric command to specify a metric value for the routes that
have been selected:
(config-route-map)#set metric
Sets the metric value. Valid range is 0 to 4294967295.
The route with the lowest MULTI_EXIT_DISC value is preferred in BGP. Refer to
Multi-Exit Discriminators (MEDs) on page 19 for information on options available in
AOS for handling MEDs received.
Refer to Example 6 on page 46 for a detailed BGP configuration example featuring the
use of the MED metric to influence which path is selected for inbound traffic to a local
network.
When specifying a MULTI_EXIT_DISC value, the attribute should be applied
outbound to a BGP neighbor. Refer to Applying a Route Map Entry to a BGP Neighbor
on page 30 for additional information.
Setting Local Preference for Inbound Routes
The local preference attribute can be used to influence the best path used to transmit information from
a local network to a private remote network. Adjusting the local preference value on inbound routes
from a remote network can affect the local router’s decision when transmitting traffic outbound to the
remote network.
The local preference attribute can be set for outbound routes, but it is only relevant in
iBGP scenarios because the local preference attribute is not retained across different
autonomous systems.
Use the set local-preference command to change the LOCAL_PREF value for selected inbound
routes:
61200860L1-29.4E Copyright © 2012 ADTRAN, Inc. 29