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Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes | Page 33
BGP: Configuring Route Maps
The effect of different combinations of clauses
A map entry could consist of:
one match clause with an action, or
no match clause and one or more set clauses, or
one match clause and one or more set clauses
Let us consider each entry type in turn.
One match clause with an action
The effect of an entry that contains a match, but no sets, will be to apply the specified action
to all update messages that match the entry. The action can be either permit or deny. This is
simply a filter—it simply lets updates through or blocks them; it does not alter updates in any
way.
If the match criterion in the match clause specifies AS path or Community and the action is
deny, then whole update messages that match the criterion will be discarded.
If the match criterion in the match clause specifies a prefix list or ACL, then only the prefixes
which match the prefix-list or ACL will be dropped. Those prefixes will be removed from the
update message, but other prefixes in the update message will not be removed.
No match clause and one or more set clauses
If an entry has no match clause, then it is assumed to match all update packets.
It is possible to add one set clause for each type of attribute that can be set. Of course, it is
not compulsory to add a clause for each of these attributes. If there is no set clause for an
attribute, the switch will simply leave it unaltered in update packets.
It is possible to specify a route map action of deny for a route map that includes a set clause,
but then there is no point in having defined any attributes to be set, as the routes are going
to be discarded anyway. However, this is a neat way of turning a clause on or off without
destroying the configuration entirely.
Note: Once you have created an entry that has no match clause, there is no point in adding
any more entries below that one, as this entry will match all packets, and the entries
below it will never be considered.
A match clause and one or more set clauses
The most general case is for an entry to have one match clause (it can never have more than
one match clause) and a number of set clauses.
Obviously enough, such an entry will apply the activities defined by the set clauses to only
those update packets or prefixes that are picked out by the match clause.