Introduction
This document provides a sample configuration on how to redirect traffic in OSPF
Requirements
It is assumed that a person reading this document is familiar with general Ospf operation.
Components used
For this document 3700 Series router with IOS 12.4(2)T loaded in it.
Network Diagram
Configuration
The configuration files are attached with this document.
For this example we will take network 10.1.23.0/24 and manipulate routing in a way that R2 is the router which is always preffered over R3 by R1.
Before we start making changes in routing table of R1:
Lets have a look at the rouring table of R1 and see how it reaches network 10.1.23.0/24:
R1#show ip route ospf
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/65] via 10.1.12.2, 00:24:06, Serial1/0.12
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/65] via 10.1.13.3, 00:24:06, Serial1/0.13
10.0.0.0/24 is subnetted, 3 subnets
O 10.1.23.0 [110/74] via 10.1.13.3, 00:24:06, Serial1/0.13
[110/74] via 10.1.12.2, 00:24:06, Serial1/0.12
Now to make sure that Router R2 is always preffered over R3, we have number of options available like, we can change bandwidth or cost of links. However, the option we are going to use involves only one command and is easy to use. The command we will be using is
max-metric router-lsa
We will configure this command on Router R3, here is the syntax:
R3#config t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#router ospf 1
R3(config-router)#max-metric router-lsa
This will configure R3 to advertise a maximum metric so that the other routers do not prefer the router as an immediate next-hop in their Shortest path first calculations.
Verify
Lets see the routing table of Router R1:
R1#show ip route ospf
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/65] via 10.1.12.2, 00:03:12, Serial1/0.12
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/65] via 10.1.13.3, 00:03:12, Serial1/0.13
10.0.0.0/24 is subnetted, 3 subnets
O 10.1.23.0 [110/74] via 10.1.12.2, 00:03:12, Serial1/0.12
We can see that R1 only uses R2 as next-hop for networl 10.1.23.0/24 now.
