The Cisco IOS Command to Ignore Oldest Path in BGP Best Path Selection

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The BGP Best Path Selection Algorithm is used by a BGP router to select the best path among multiple possible paths to a destination network. When two external paths are received from different EBGP neighbors, the router may face a tie between two paths that are equally good based on the BGP decision criteria.

In such a scenario, the oldest path received can be used as a tiebreak to select the best path. This is because the router assumes that the older path is more stable and less likely to be flapping than the newer path.

However, there may be cases where the age of the path should not be used as a tiebreaker, and the router should instead check the next decision criteria. This can be achieved by using the "bgp best path" command with one of the following options:

A. "bgp best path med missing-as-worst": This option makes the router treat a missing MED value as the worst possible value, which means that a path with a missing MED will never be selected as the best path.

B. "bgp best path compare-routerid": This option makes the router compare the router IDs of the BGP speakers that advertised the paths, instead of using the age of the paths as a tiebreaker.

C. "bgp best path cost-community ignore": This option makes the router ignore any cost community values that may be present in the paths, and move on to the next decision criteria.

D. "bgp best path as-path ignore": This option makes the router ignore the AS path length as a decision criteria, and move on to the next criteria.

In summary, the "bgp best path" command can be used with different options to modify the BGP Best Path Selection Algorithm and select the best path based on different criteria. In this particular case, the option that would ignore the age of the paths as a tiebreaker is "bgp best path compare-routerid".