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Configuring BGP Setting Global BGP Parameters
OmniSwitch 6800/6850/9000 Advanced Routing Configuration Guide December 2007 page 4-23
Controlling the use of MED Values
The Multi Exit Discriminator, or MED, is used by border routers (i.e., BGP speakers with links to neigh-
boring autonomous systems) to help choose between multiple entry and exit points for an autonomous
system. It is only relevant when an AS has more than one connection to a neighboring AS. If all other
factors are equal, the path with the lowest MED value takes preference over other paths to the neighbor
AS.
If received on external links, the MED may be propagated over internal links to other BGP speakers in the
same AS. However, the MED is never propagated to speakers in a neighboring AS. The MED attribute
indicates the weight of a particular exit point from an AS. Some exit points may be given a better MED
value because they lead to higher speed connections.
The Alcatel-Lucent implementation of BGP allows you to control MED values in the following ways:
• Compare MED values for external ASs
• Insert a MED value in routes that do not contain MEDs
The following two sections describe these MED control features.
Enabling MED Comparison for External Peers
By default, BGP only compares MEDs from peers within the same autonomous system when selecting
routes. However, you can configure BGP to compare MEDs values received from external peers, or other
autonomous systems. To enable MED comparison of external peers specify:
-> ip bgp always-compare-med
This command requires that you first disable the BGP protocol. If BGP were already enabled, you would
actually need to issue two commands to disable MED comparison:
-> ip bgp status disable
-> no ip bgp always-compare-med
Inserting Missing MED Values
A MED value may be missing in a route received from an external peer. Your can specify how a missing
MED in an external BGP path is to be treated for route selection purposes. The default behavior is to treat
missing MEDs as zero (best). The ip bgp bestpath med missing-as-worst command allows you to treat
missing MEDs as 2
32-1
(worst) for compatibility reasons.
To change the missing MED value from worst to best, enter the following command:
-> ip bgp bestpath med missing-as-worst