One Way Redistribution - SubOptimal Problem

The use of a routing protocol to advertise routes that are learned by some other means, such as by another routing protocol, static routes, or directly connected routes, is called redistribution.

One way redistribution is easy to make but some sub-optimal routing can happen.

Network Example:

Steps to reproduce this lab:

R3(config)# router eigrp 1

R3(config-router)# redistri connect

R1(config)# router ospf 1

R1(config-router)# redistr eigrp 1 subnets

* Problem:

After the end of the day, you discovery that R2 chose the R1to get into 10.1.1.0 network. Not good, right? Why he do that? Because after the redistribution of the EIGRP into OSPF, the 10.1.1.0 network is advertised with the distance of 110 and is better that 170 of the EIGRP. See How to Fix that.

• Before redistribution

R2# sh ip route

...

Gateway of last resort is not set

C    192.168.10.0/24 is directly connected, Serial0/1

D    192.168.11.0/24 [90/2681856] via 192.168.20.3, 00:02:33, Serial0/0

C    192.168.20.0/24 is directly connected, Serial0/0

     10.1.1.0/24 is subnetted, 1 subnets

D EX    10.1.1.0 [170/2297856] via 192.168.20.3, 00:00:22, Serial0/0

R2# ping 10.1.1.1 rep 1

Type escape sequence to abort.

Sending 1, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:

!

Success rate is 100 percent (1/1), round-trip min/avg/max = 1/1/1 ms

R2# traceroute 10.1.1.1

Type escape sequence to abort.

Tracing the route to 10.1.1.1

  1 192.168.20.3 16 msec 36 msec 16 msec

• But, after the redistribution into OSPF..

R2# sh ip route 

...

Gateway of last resort is not set

C    192.168.10.0/24 is directly connected, Serial0/1

D    192.168.11.0/24 [90/2681856] via 192.168.20.3, 00:05:09, Serial0/0

C    192.168.20.0/24 is directly connected, Serial0/0

     10.1.1.0/24 is subnetted, 1 subnets

O E2    10.1.1.0 [110/20] via 192.168.10.1, 00:00:05, Serial0/1

R2# trace 10.1.1.1

Type escape sequence to abort.

Tracing the route to 10.1.1.1

  1 192.168.10.1 4 msec 40 msec 28 msec

  2  *  *  * 

  3  *  *  * 

  4  *  *  * 

  5  *  *  * 

  6  *  *  * 

  7  *  *

It's not good ... let's fix it! : )

R2# conf t

R2(config)#

R2(config)# access-list 1 permit 10.1.1.0 0.0.0.255

R2(config)# router ospf 1

R2(config-router)# distance 171 0.0.0.0 255.255.255.255 1

R2(config-router)#^Z

R2# sh ip route 

...

Gateway of last resort is not set

C    192.168.10.0/24 is directly connected, Serial0/1

D    192.168.11.0/24 [90/2681856] via 192.168.20.3, 00:07:49, Serial0/0

C    192.168.20.0/24 is directly connected, Serial0/0

     10.1.1.0/24 is subnetted, 1 subnets

D EX    10.1.1.0 [170/2297856] via 192.168.20.3, 00:00:02, Serial0/0

look nice to me.

R2# trace 10.0.0.1

Type escape sequence to abort.

Tracing the route to 10.1.1.1

  1 192.168.20.3 12 msec 20 msec 32 msec

Sub-Optimal routing solved! Just for today..

Some tips to solve routing redistribution problems

• - Redistribute only internal routes from routing protocol X to routing protocol Y.
• - Tag routes on routers doing redistribution.
• - Increase the metric for redistributes routes so internal routes are used instead of the redistributed ones.
• - Change the administrative distance.

[*] Cisco Documentation

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