There are times that you don’t want to redistribute all routes from one protocol into another. Route maps can help you with this, allowing you to select which routes to redistribute. Also, you can use route maps to manipulate routes before you redistribute them.
Consider the following brief IOS code example:
router ospf 1
router eigrp 1
redistribute ospf 1 route-map ospf-into-eigrp
default-metric 1544 5 1 1 1
route-map ospf-into-eigrp permit 10
match ip next-hop GW_10-254-254-254
set tag 50
ip access-list standard GW_10-254-254-254
In this example, OSPF routes are candidates for redistribution into EIGRP AS 1. Their candidacy is governed by a route map called “ospf-into-eigrp”. The ospf-into-eigrp route map will redistribute routes with a next-hop matching access control entries in the access list “GW_10-254-254-254”. That ACL permits 10.254.254.254 – ergo, routes will a next hop of 10.254.254.254 will be redistributed. In addition, the route tag field will be populated with “50”. When these routes are redistributed, the default-metric will be used of 1544 for bandwidth, and 5 for delay. The remaining 1’s in the default metric are irrelevant, unless your network is configured for non-default K-values.
There’s a lot of other things that you could do with the route-maps, including set metrics, permit based on advertising router, and permit based on route-type, but you get the idea. Remember that at the bottom of a route-map is an implicit “deny any any”. Therefore, in the above example, routes that do NOT have a next-hop of 10.254.254.254 would not be redistributed.