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.

Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes Page 33

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Allied Telesis X8100, x908 manual The effect of different combinations of clauses, One match clause with an action
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X8100, x908 specifications

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