Cisco CCNA – Routing Redistribution

Figure -1

The example shown above may be encountered when two company’s merge such as Company A and Company B with different networks and routing domains upon the merger also merge the IT infrastructure.

Each routing protocols has its own metric. For example RIP expresses routes in terms of hop count, while EIGRP’s metric uses a composite metric (bandwidth, delay, load, reliability and MTU). With route redistribution, the metric of one protocol has to be translated into a metric which the other protocol can understand. Setting a default, or seed metric does this. The metric configured must be understandable to the receiving protocol. So the router performing the redistribution must assign a metric to the redistributed routes. Router B when redistributing EIGRP routes into OSPF must assign those EIGRP routes that are redistributed into OSPF the metric used by OSPF, thus in this case all EIGRP routes that are present in Router B’s routing table when redistributed into OSPF must be assigned the cost metric.

If a router that is running more than one protocol and learns the same route from each protocol, then the router will use the administrative distance to decide which route to prefer. Administrative distance is a measure of the trustworthiness of the source of the routing information. Administrative distance has only local significance, and is not advertised in routing updates. The smaller the administrative distance value, the more reliable the protocol. For example, if a router receives a route to a certain network from both Open Shortest Path First (OSPF) (default administrative distance – 110) and Routing Information Protocol (RIP) (default administrative distance – 120), the router chooses OSPF because it is more reliable. This means the router adds the OSPF version of the route to the routing table.

Table below lists the default administrative distance values of the protocols in Cisco IOS.

Configuration

We will use the network shown in figure-1 to learn how to configure routing redistribution. Router B will be configured to redistribute OSPF routes into EIGRP, and EIGRP routes into OSPF. We have assumed that the basic routing protocol configuration on all routers is present

RB(config)# router ospf 100

RB(config-router)# redistribute eigrp 10 subnets

RB(config)# router eigrp 10

RB(config-router)# redistribute ospf 100 metric 10000 10 255 1 1500

The configuration above redistributes the EIGRP 10 process into OSPF process 100. The subnets keyword is used to redistribute subnets, if it is not specified OSPF will only redistribute classfull networks.

OSPF process 100 is then redistributed in EIGRP process 10 with the seed metric values specified.

Note:- When redistributing Routing Protocol A into Routing Protocol B, only those routes present for the protocol A in the routing table will be redistributed into protocol B

This brings us to the end of this article in which we covered basic routing redistribution. Redistribution must be carefully planned and if not required it must be avoided as it leads various issues and problems such as routing loops