About Route Maps
Route maps are used when redistributing routes into an OSPF, RIP, EIGRP or BGP routing process. They are also used when generating a default route into an OSPF routing process. A route map defines which of the routes from the specified routing protocol are allowed to be redistributed into the target routing process.
Route maps have many features in common with widely known ACLs. These are some of the traits common to both:
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They are an ordered sequence of individual statements, and each has a permit or deny result. Evaluation of an ACL or a route map consists of a list scan, in a predetermined order, and an evaluation of the criteria of each statement that matches. A list scan is aborted once the first statement match is found and an action associated with the statement match is performed.
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They are generic mechanisms. Criteria matches and match interpretation are dictated by the way that they are applied and the feature that uses them. The same route map applied to different features might be interpreted differently.
These are some of the differences between route maps and ACLs:
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Route maps are more flexible than ACLs and can verify routes based on criteria which ACLs can not verify. For example, a route map can verify if the type of route is internal.
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Each ACL ends with an implicit deny statement, by design convention. If the end of a route map is reached during matching attempts, the result depends on the specific application of the route map. Route maps that are applied to redistribution behave the same way as ACLs: if the route does not match any clause in a route map then the route redistribution is denied, as if the route map contained a deny statement at the end.
Permit and Deny Clauses
Route maps can have permit and deny clauses. The deny clause rejects route matches from redistribution. You can use an ACL as the matching criterion in the route map. Because ACLs also have permit and deny clauses, the following rules apply when a packet matches the ACL:
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ACL permit + route map permit: routes are redistributed.
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ACL permit + route map deny: routes are not redistributed.
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ACL deny + route map permit or deny: the route map clause is not matched, and the next route-map clause is evaluated.
Match and Set Clause Values
Each route map clause has two types of values:
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A match value selects routes to which this clause should be applied.
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A set value modifies information that will be redistributed into the target protocol.
For each route that is being redistributed, the router first evaluates the match criteria of a clause in the route map. If the match criteria succeeds, then the route is redistributed or rejected as dictated by the permit or deny clause, and some of its attributes might be modified by the values set from the set commands. If the match criteria fail, then this clause is not applicable to the route, and the software proceeds to evaluate the route against the next clause in the route map. Scanning of the route map continues until a clause is found that matches the route or until the end of the route map is reached.
A match or set value in each clause can be missed or repeated several times, if one of these conditions exists:
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If several match entries are present in a clause, all must succeed for a given route in order for that route to match the clause (in other words, the logical AND algorithm is applied for multiple match commands).
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If a match entry refers to several objects in one entry, either of them should match (the logical OR algorithm is applied).
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If a match entry is not present, all routes match the clause.
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If a set entry is not present in a route map permit clause, then the route is redistributed without modification of its current attributes.
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Do not configure a set entry in a route map deny clause because the deny clause prohibits route redistribution—there is no information to modify. |
A route map clause without a match or set entry does perform an action. An empty permit clause allows a redistribution of the remaining routes without modification. An empty deny clause does not allow a redistribution of other routes (this is the default action if a route map is completely scanned, but no explicit match is found).
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