Route Redistribution

What is route redistribution?Why is it required in network?Where is it implemented in the network?

How is it implemented Problems due to route redistribution?

routing structure of a large enterprise network typically consists of multiple domains or routing instances due to:◦ Company acquisitions◦ departments administered by different teams◦ multi-vendor equipments◦ Intentional creation of separate routing instances

to filter routes limit reachability and enforce policies

Careful and thoughtful migration from one routing protocol to another

The new routing protocol will most likely have different requirements and capabilities from the old one.

It is important for network administrators to understand what must be changed and to create a detailed plan before making any changes

Routers within one routing instance typically run the same routing protocol to fully share reachability information

do not exchange routing information with routers in other routing instances

Figure 1. It consists of two routing instances. ◦ Routers in the RIP instance do not

have visibility of the addresses and subnet prefixes in the OSPF instance and vice versa.

RIP OSPF

D

E C

B

A G

F

H

• Each routing protocol defines a metric for each route. • RIP – 120, OSPF – 110, EIGRP - 90

• When a router redistributes routes from one routing domain to another this information cannot be translated from one routing protocol to another. • A RIP hop cannot be dynamically recalculated to

an OSPF cost by the router doing redistribution. • A seed metric is used to artificially set the

distance, cost, etc., to each redistributed network from the redistribution point.

Protocol Default Seed Metrics

RIP Infinity

IGRP/EIGRP Infinity

OSPF 20 for all except BGP, which is 1

IS-IS 0

BGP BGP metric is set to IGP metric value

Locate the boundary router between two routing processes.

Determine which routing process is the core or backbone process

Determine which routing process is the edge or migration process

Select a method for injecting the required edge protocol routes into the core.

RtrA(config)# router ospf 1RtrA(config-router)# redistribute eigrp ?

<1-65535> Autonomous system numberRtrA(config-router)# redistribute eigrp 100 ?

metric Metric for redistributed routes metric-type OSPF/IS-IS exterior metric type for redistributed routes route-map Route map reference subnets Consider subnets for redistribution into OSPF tag Set tag for routes redistributed into OSPF … <cr>

Router C is using a seed metric of 30 inside the OSPF routing process to redistributed RIP routes. ◦ ROUTER C (config-router) redistribute rip subnets metric

30. The link cost of the WAN link to router D is 100. The cost for networks 1.0.0.0, 2.0.0.0, and 3.0.0.0 in router D is

the seed metric (30) plus the link cost (100) = 130.

Bandwidth in kilobytes = 10000 Delay in tens of microseconds = 100 Reliability = 255 (maximum) Load = 1 (minimum) MTU = 1,500 bytes

router ospf 1 redistribute rip metric 10000 metric-type 1 subnets network 172.31.0.0 0.0.255.255 area 0!router rip version 2 redistribute ospf 1 metric 5 network 10.0.0.0 no auto-summary

router ospf 1 redistribute rip metric 10000 metric-type 1 subnets network 172.31.3.2 0.0.0.0 area 0!router rip version 2 redistribute ospf 1 metric 5 network 10.0.0.0 no auto-summary

Router P3R1

Router P3R2

hostname P3R1!router ospf 1 redistribute rip metric 10000 metric-type 1 subnets network 172.31.0.0 0.0.255.255 area 0 distance 125 0.0.0.0 255.255.255.255 64 !router rip version 2 redistribute ospf 1 metric 5 network 10.0.0.0 no auto-summary!

hostname P3R2!router ospf 1redistribute rip metric 10000 metric-type 1 subnets network 172.31.3.2 0.0.0.0 area 0 distance 125 0.0.0.0 255.255.255.255 64!router rip version 2 redistribute ospf 1 metric 5 network 10.0.0.0 no auto-summary!

hostname P3R1!router ospf 1 redistribute rip metric 10000 metric-type 1 subnets network 172.31.0.0 0.0.255.255 area 0 distance 125 0.0.0.0 255.255.255.255 64 !router rip version 2 redistribute ospf 1 metric 5 network 10.0.0.0 no auto-summary!

hostname P3R2!router ospf 1redistribute rip metric 10000 metric-type 1 subnets network 172.31.3.2 0.0.0.0 area 0 distance 125 0.0.0.0 255.255.255.255 64!router rip version 2 redistribute ospf 1 metric 5 network 10.0.0.0 no auto-summary!

Be very familiar with your network BEFORE implementing redistribution

Focus on routers with redundant paths

Make sure no path information is lost when using the distance command