Using Cisco IOS Route Maps To Control Path Selection

by Tony DeSimone

What is a route map?

Cisco IOS route maps can be used for a variety of different purposes. They offer a network engineer a lot of control in manipulating path selection. Simply put – route maps provide scripting-type logic which allows routers to take certain actions if the information in a packet matches the criteria defined by the network engineer.

What can route maps do? 

Among other uses, route maps are commonly used when redistributing routes into  a routing process like OSPF (Open Shortest Path First) or EIGRP (Enhanced Interior Gateway Routing Protocol). With redistribution, route maps are used to define which of the routes from a particular routing protocol are allowed to be redistributed into the target routing process as well as to manipulate metrics during the redistribution.

Route maps are also the main component of policy-based routing which allows an administrator to override the path defined in the routing table.

For example, the following route map is used to perform policy-based routing. The configuration is designed to take traffic sourced from the 192.168.1.0 network that enters the router on FastEthernet 0/1 and send it to the next hop router of 10.1.1.1:

route-map BUBBA permit 10
  match ip address 22
  set ip next-hop 10.1.1.1

access-list 22 permit 192.168.1.0  0.0.0.255

interface FastEthernet 0/1
ip policy route-map BUBBA

Many times, as is the case in this example, the criteria that you would like the route map to take action on is based upon any traffic defined in an access-list.

Did you notice that there is a choice of permit/deny on the route-map statement as well as a choice of permit/deny in the access-list that is referenced in the route map?

With all the possible combinations (such as a permit and a permit, or a permit and a deny, or a deny and a deny, etc…) it becomes difficult to determine the outcome of the logic.

My approach to route maps is to always use a permit on the referenced access list. Look at the access-list as simply defining the traffic.  Then use the permit or deny of the route map to determine to allow or not allow that specified traffic.

This approach may not fit all the possible logic you wish to incorporate into a route map, but it has made things much easier for me.

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