Information Technology Reference
In-Depth Information
When both the
set community
and
set comm-list delete
commands are configured in the
same instance of a route map, the delete operation is performed before the set operation.
NOTE
Route Maps
A route map is a flexible and powerful way to set BGP policies. It can set and reset both
prefixes and BGP attributes based on predefined conditions. A route map is often used to
define policies toward a BGP peer or during route generation. A route map can filter updates
based on prefix, AS_PATH, communities, metrics, next hop, ORIGIN, LOCAL_PREF,
WEIGHT, and so on. A route map often uses policy control lists to define BGP policies.
A route map is a named group of filters consisting of one or more instances. Each instance
is identified by a unique sequence number that determines the order of processing. Instanc-
es are applied sequentially. If a match is found, the rest of the route map is skipped. If
the route map is finished without a match, a deny action is performed. When used in the
neighbor
command, only one route map per type per direction is allowed for each neighbor.
Within each instance, you can set conditions using the
match
clause and set actions using
the
set
clause. Example 4-11 shows a simple route map named Set-comm, which resets
communities to 200:100 when updates are originated from AS 100.
Example 4-11
Simple Route Map Example
ip as-path access-list 1 permit _100$
!
route-map Set-comm permit 10
match as-path 1
set community 200:100
route-map Set-comm permit 20
The second instance (with sequence number 20) is important, because without it, all other
updates that don't match the first instance are not accepted. When no match clause is spec-
ified under an instance, the result is to permit any. This instance basically means that no
action should be taken for prefixes that do not match the conditions in the first instance.
The
deny
keyword in a route map is equivalent to a
no
keyword for other commands, but
it does not necessarily indicate to deny something. The exact meaning depends on the route
map's purpose. For example, if a route map is to suppress a route,
deny
is used to unsup-
press that route. The same concept also applies to other forms of filtering of BGP prefixes
and attributes.
NOTE
