Route maps come in handy during IP routing redistribution as a way to select (or filter) some routes from a given routing source and redistribute them into another routing source.
There are three main components of route-map configuration in this context, i.e.
Let’s now try this out with an example network topology. Assuming we have properly configured OSPF and EIGRP on each routing domain and now we’re tasked to mutually redistributing those routes using route-maps.
The objective is to only redistribute networks 192.168.10.0/24 and 192.168.20.0/24 into OSPF and advertise them as external type 1 (E1) routes with an external metric of 20. In order to achieve our goal, we need to configure and apply route-map on R2, the redistribution point.
R2(config)#access-list 10 permit 192.168.10.0 0.0.0.255
R2(config)#access-list 10 permit 192.168.20.0 0.0.0.255
R2(config)#route-map CCIEin8Weeks permit 10
R2(config-route-map)#match ip address 10
R2(config-route-map)#set metric 20
R2(config-route-map)#set metric-type type-1
R2(config)#router ospf 10
R2(config)#redistribute eigrp 100 subnets route-map CCIEin8Weeks
Please note that 192.168.30.0/24 and 192.168.40.0/24 will not be redistributed because of the implicit deny any at the end of the route map ACL.
In a two-way multipoint redistribution, you can use route-map tagging for route selection and filtering.
R2(config)#route-map EIGRP-to-OSPF deny 10
R2(config-route-map)#match tag 66
R2(config-route-map)#route-map EIGRP-to-OSPF permit 20
R2(config-route-map)#set tag 33
R2(config-route-map)#router ospf 10
R2(config-router)#redistribute eigrp 100 subnets route-map EIGRP-to-OSPF
Please note that you will need to apply a similar route-map configuration to R3 in order to filter OSPF to EIGRP routes.
Route maps are powerful tools when it comes to redistribution of routes. They allow network engineers to apply very fine but specific changes to routing information when it is redistributed between routing protocols. While troubleshooting route-map misconfigurations in routing applications, you’re likely to find issues related to suboptimal routing or even traffic black holing.