Redistribution Techniques

Route 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, we only use redistribute command only for one protocol. 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

 

In the 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 10.10.0.1/32 and we have redistributed that subnet in RIP. 

R2(config)#ip route 10.10.0.1 255.255.255.255 10.10.12.1

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

In this method we have to redistribute all routes between the two routing protocols.

In the 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:

Route Redistribution Key Points

• The redistribute command is configured under the routing process that is to receive the redistributed routes. The metric and redistribute rules should be followed based on the protocol that is receiving the routes and not with respect to the protocol that is sending the routes.
  

• The routes are only redistributed from routing protocols that support the same protocol stack. For instance, you can redistribute between  IP RIP and OSPF because they both support the TCP-IP protocol stack. Redistribution cannon happen between Internetwork Packet Exchange (IPX) RIP and OSPF because IPX RIP supports the IPX/Sequenced Packet Exchange (SPX) stack and OSPF does not. Although there are different protocol-dependent modules of EIGRP for IP, IPX, and AppleTalk, routes cannot be redistributed between them, because each protocol-dependent module supports a different protocol stack.
  

• The way used to configure redistribution varies among combinations of routing
  protocols. For example, there is no need of manual redistribution between IGRP and EIGRP when they belong to same Autonomous Number (AS) as the redistribution will occur automatically, however, redistribution must be configured between all other routing protocols. Some routing protocols require a metric to be configured during redistribution, but others do not.
  

• The routes that you are considering for redistribution must be in present the routing table of the advertising or source protocol 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 value. 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.
  

• Redistribution is always performed in the outbound direction; 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.
   

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