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Common Configuration Problem with the Longest-Match Lookup

A common problem when defining a route list is including a shorter prefix that you want to match with a longer, similar prefix in the same list. For example, imagine that the prefix 192.168.254.0/24 is compared against the following route list:



route-filter 192.168.0.0/16 orlonger;
route-filter 192.168.254.0/23 exact;

Because the policy framework software performs longest-match lookup, the prefix 192.168.254.0/23 is determined to be the longest prefix. An exact match does not occur between 192.168.254.0/24 and 192.168.254.0/23 exact. The software determines that the term does not match and goes on to the next term or routing policy, if present, or takes the accept or reject action specified by the default policy. (For more information about the default routing policies, see Default Routing Policies and Actions.) The shorter prefix 192.168.0.0/16 orlonger that you wanted to match is inadvertently ignored.

One solution to this problem is to remove the prefix 192.168.0.0/16 orlonger from the route list in this term and move it to a previous term where it is the only prefix or the longest prefix in the list.

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