How to Use a Loopback Interface to Create an ABR Router

OSPF requires that all traffic between non-backbone routers pass through the backbone area (Area 0) in order to avoid routing loops.  This document describes how to advertise routes between on-backbone routers.

In the example below, Router A has an interface in Area 23, Router B has interfaces in Area 23 and Area 24 and Router C has an interface in Area 24.  All are non-backbone routers.  The goal is to advertise routes between routers A and C.

This can be accomplished by configuring a loopback address on Router B and assigning it to Area 0.  The loopback and L3 interfaces in Areas 23 and 24 will be assigned to the same Virtual Router.  This will turn Router B into an Area Backbone Router and Routers C and A will be able to exchange routes.  The loopback interface will be configured as passive, preventing it from forming adjacencies.

Under the tab Network > Interfaces, create the L3 interfaces using the appropriate addressing and zones.

Create the Loopback address.  Assign any IP address and any zone.

Under the tab Network > Virtual Routers, create the profile for the L3 and loopback interfaces.

On the OSPF tab, click New for each area and configure the areas for the L3 interfaces and loopback, enable OSPF and check the box to reject the default route.

When configuring the loopback interface, under the Interface tab, check the Passive box.

Verify the router is now an ABR with the following CLI command:

admin@PA-2050> show routing protocol ospf summary

owner: panagent