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Configuring the MED Attribute

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, 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.

Figure 35: Configuring the MED

Image g013192.gif

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 

host2(config-router)#neighbor 10.6.6.1 remote-as
4 

host2(config-router)#neighbor 10.6.6.1 route-map
10 out 

host2(config-router)#network 192.168.33.0
mask 255.255.255.0 

host2(config-router)#exit 

host2(config)#route-map 10 permit 10 

host2(config-route-map)#set metric 50

The following commands configure router Nice:



host3(config)#router bgp 73 

host3(config-router)#neighbor 10.3.3.1 remote-as
303 

host3(config-router)#neighbor 10.3.3.1 route-map
10 out 

host3(config-router)#neighbor 10.2.25.2 remote-as
73 

host3(config-router)#network 172.19.0.0 

host3(config-router)#exit 

host3(config)#route-map 10 permit 10 

host3(config-route-map)#set metric 100

The following commands configure router Dublin:



host4(config)#router bgp 4 

host4(config-router)#neighbor 10.5.5.1 remote-as
303 

host4(config-router)#neighbor 10.5.5.1 route-map
10 out 

host4(config-router)#neighbor 10.6.6.2 remote-as
73 

host4(config-router)#network 172.14.27.0 mask
255.255.255.0 

host4(config-router)#exit 

host4(config)#route-map 10 permit 10 

host4(config-route-map)#set metric 25

Router London receives updates regarding route 192.168.33.0/24 from both router Nice and router Paris. Router London compares the MED values received from the two routers: Router Nice advertises a MED of 100 for the route, whereas router Paris advertises a MED of 50. On this basis, router London prefers the path through router Paris.

Because BGP by default compares only MED attributes of routes coming from the same AS, router London can compare only the MED attributes for route 192.168.33.0/24 that it received from routers Paris and Nice. It cannot compare the MED received from router Dublin, because router Dublin is in a different AS than routers Paris and Nice.

However, you can use the bgp always-compare-med command to configure router London to take into account the MED attribute from router Dublin as follows:



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 

host1(config-router)#bgp always-compare-med

Router Dublin advertises a MED of 25 for route 192.168.33.0/24, which is lower—more preferred—than the MED advertised by router Paris or router Nice. However, the AS path for the route through router Dublin is longer than that through router Paris. The AS path is the same length for router Paris and router Nice, but the MED advertised by router Paris is lower than that advertised by router Nice. Consequently, router London prefers the path through router Paris.

Suppose, however that router Dublin was not configured to set the MED for route 192.168.33.0/24 in its outbound route map 10. Would router London receive a MED of 50 passed along by router Paris through router Dublin? No, because the MED attribute is nontransitive. Router Dublin does not transmit any MED that it receives. A MED is only of value to a direct peer.

bgp always-compare-med

Use to enable the comparison of the MED for paths from neighbors in different ASs.
Unless you specify the bgp always-compare-med command, the router compares MED attributes only for paths from external neighbors that are in the same AS.
The BGP path decision algorithm selects a lower MED value over a higher one.
Unlike local preferences, the MED attribute is exchanged between ASs, but does not leave the AS.
The value is used for decision making within the AS only.
When BGP propagates a route received from outside the AS to another AS, it removes the MED.
Examplehost1(config-router)#bgp always-compare-med
Changes apply automatically whenever BGP subsequently runs the best-path decision process for a destination prefix; that is, whenever a best route is picked for a given prefix.
To force BGP to run the decision process on routes already received, you must use the clear ip bgp command to perform an inbound soft clear or hard clear of the current BGP session.
Use the no version to disable the feature.
See bgp always-compare-med.

set metric

Use to set the metric value—for BGP, the MED—for a route.
Sets an absolute metric. You cannot use both an absolute metric and a relative metric within the same route map sequence. Setting either metric overrides any previously configured value.
Examplehost1(config)#route-map nyc1 permit 10 host1(config-route-map)#set metric 10
Use the no version to delete the set clause from a route map.
See set metric.

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