Configuring The Med Attribute; Figure 35: Configuring The Med - Juniper JUNOSE 11.2.X BGP AND MPLS Configuration Manual

Configuring the MED Attribute

Figure 35: Configuring the MED

Copyright © 2010, Juniper Networks, Inc.
If two ASs connect to each other in more than one place, one link or path might be a
better choice to reach a particular prefix within or behind one of the ASs. The MED value
is a metric expressing a degree of preference for a particular path. Lower MED values are
preferred.
Whereas the Local Preference attribute is used only within an AS (to select an exit point),
the MED attribute is exchanged between ASs. A router in one AS sends the MED to inform
a router in another AS which path the second router should use to reach particular
destinations. If you are the administrator of the second AS, you must therefore trust that
the router in the first AS is providing information that is truly beneficial to your AS.
You configure the MED on the sending router by using the set metric command in an
outbound route map. Unless configured otherwise, a receiving router compares MED
attributes only for paths from external neighbors that are members of the same AS. If
you want MED attributes from neighbors in different ASs to be compared, you must issue
the bgp always-compare-med command.
In Figure 35 on page 119, router London in AS 303 can reach 192.168.33.0/24 in AS 73
through router Paris or through router Nice to router Paris.
The following commands configure router London:
host1(config)#router bgp 303
host1(config-router)#neighbor 10.4.4.2 remote-as 73
host1(config-router)#neighbor 10.3.3.2 remote-as 73
host1(config-router)#neighbor 10.5.5.2 remote-as 4
host1(config-router)#network 122.28.8.0 mask 255.255.248.0
The following commands configure router Paris:
host2(config)#router bgp 73
host2(config-router)#neighbor 10.4.4.1 remote-as 303
host2(config-router)#neighbor 10.4.4.1 route-map 10 out
host2(config-router)#neighbor 10.2.25.1 remote-as 73
Chapter 1: Configuring BGP Routing
119