Redistribution Techniques

Redistribution is technique using which the boundary routers connecting different routing domains can exchange and advertise routing information. There are 2 methods of route redistribution:
  • One way redistribution: This method is also called one sided redistribution. In this method, a default route is propagated into one routing protocol  and on the other side, it redistributes only the networks learned from that routing protocol into the other routing protocol.
  • Two way redistribution: In this method we have to redistribute all routes between the two routing protocols.
One Way Redistribution Config Example:

One-Way Route Redistribution

In above topology, R2 and R3 are running RIPv2 and no routing protocol is running on R1. So in order to complete the routing, we have to put a static route on R2 for R1's Loopback0 subnet and we have redistributed that subnet in RIP.

R2(config)#ip route

R2(config)#router rip
R2(config-router)#redistribute static metric 3

Kindly also note that we should be careful our choice of configuring static or default route on a non stub router such as R2. Any incorrect default routing can lead to network issues.My recommendation is to use static routing. In order to avoid configuring multiple static routes on R2 for R1's prefixes, we can try to summarize the prefixes where possible.

Two Way Redistribution Config Example:

Two-Way Route Redistribution

In above topology, R1 is OSPF domain and R3 is in RIP domain.  R2 is a boundary router running both OSPF and RIP. So in order to exchange the routes between 2 domains, we need to do a two way redistribution of routes on R2 using below config:

R2(config)#router ospf 1
R2(config-router)#redistribute rip subnets

R2(config-router)#router rip
R2(config-router)#redistribute ospf 1 metric 3

Other Imp information about Redistribution:

Redistribution is always performed outbound; the router doing redistribution does not change its routing table. For example, when redistribution between OSPF and EIGRP is configured, the OSPF process on the boundary router takes the EIGRP routes in the routing table and advertises them as OSPF routes to its OSPF neighbors. Likewise, the EIGRP process on the boundary router takes the OSPF routes in the routing table and advertises them as EIGRP routes to its EIGRP neighbors. With this redistribution, both autonomous systems know about the routes of the other, and each autonomous system can then make informed routing decisions for these networks. The boundary router’s neighbors see the redistributed routes as external routes. In this example, if a packet destined for one of the networks in the OSPF domain arrives from the EIGRP autonomous system, the boundary router must have the OSPF routes for the networks in the OSPF domain in its routing table to be able to forward the traffic.

Also, routes must be in the routing table for them to be redistributed. This requirement might seem self-evident, but it can be a source of confusion. For example, if a router learns about a network via EIGRP and OSPF, only the EIGRP route is put in the routing table because it has a lower administrative distance. Suppose RIP is also running on this router, and you want to redistribute OSPF routes into RIP. That network is not redistributed into RIP, because it is placed in the routing table as an EIGRP route, not as an OSPF route.

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