Comparing MED Values Within a Confederation

A BGP speaker within a confederation of sub-ASs might need to compare routes to determine the best path to a destination. By default, BGP does not use the MED value when comparing routes originated in different sub-ASs within the confederation to which the BGP speaker belongs. (Within the confederation, routes learned from different sub-ASs are treated as having originated in different places.) You can use the bgp bestpath med confed command to force MED values to be taken into account within a confederation.

bgp bestpath med confed

• Use to specify that BGP take into account the MED when comparing routes originated in different sub-ASs within the confederation to which the BGP speaker belongs.
• This command does not affect the comparison of routes that are originated in other ASs and does not affect the comparison of routes that are originated in other confederations.
• Example

  host1(config-router)#bgp bestpath med confed

• 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 restore the default, where the MED is not taken into account.

Suppose a BGP speaker has three routes to prefix 10.10.0.0/16:

• Route 1 is originated by sub-AS 1 inside the confederation.
• Route 2 is originated by sub-AS 2 inside the confederation.
• Route 3 is originated by AS 3 outside the confederation.
• See bgp bestpath med confed.

BGP compares these routes to each other to determine the best path to the prefix. If you have issued the bgp bestpath med confed command, BGP takes into account the MED when comparing Route 1 with Route 2. However, BGP does not take into account the MED when comparing Route 3 with either Route 1 or Route 2, because Route 3 originates outside the confederation.

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