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Notes:
In the example above, assuming a full iBGP mesh inside AS1 and that all routes have the same local-pref value, router R8 will receive four paths to reach router R0 :
One path going via R5 in AS2 and received with MED=9
One path going via R3 in AS3 and received with MED=20
One path going via R2 in AS2 and received with MED=0
One path going via R4 in AS3 and received with MED=21
The local-pref and AS-Path steps of the decision process will not remove any path from consideration.
The MED step of the BGP decision process will select, from each neighboring AS, the paths with the smallest MED, namely :
One path going via R2 in AS2 and received with MED=0
One path going via R3 in AS3 and received with MED=20
Then, the closest nexthop step of the BGP decision process will select as best path the path that leaves AS1 router R7, i.e. :
One path going via R3 in AS3 and received with MED=20
This is the standardized processing of the MED attribute in BGP4. As always with BGP4 implementations, some implementations allow operators to :
Ignore the MED values from a given peer
Process all MED values without considering the AS from which the MED value was learned
in this case, the path via R5 would be selected
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