Thursday, 4 July 2013

OSPF Multi-Area with Stub Areas and Authentication

OSPF Multi-Area with Stub Areas and Authentication



  • Configure multiple-area OSPF
  • Verify multiple-area behavior
  • Configure stub, totally stubby, and not so stubby areas
  • Configure authentication

Configure multiple-area OSPF

For the OSPF configuration I will create area 0 and area 23. Items in red will be area 23, items in black will be part of Area 0.

If we take a look at the routing table onf CHI, we can see the routes to OMEGA are inter area routes.

PSI has no inter area routes as it is in both areas which classes it as an ABR.

Stub Areas

I am now going to make area 23 a stub area. This will break adjacency until i configure the other router.

The adjacency messages confirm that they are neighbours again, but you can double check with the sh ip ospf neighbor command;

If we take a look at OMEGA's routing table we can see that it now has a default route pointing to PSI. As a stub area does not get any external routes. As stub area receives a default route and OSPF inter area routes.

We can check what areas are configured on a router by using the show ip opsf command. Where we can see that PSI is in both Area 0 (backbone) and area 23 a stub area.

Totally Stubby Areas

A totally stubby area ABR only allows in a single default route from the backbone. To accomplish this the command area 23 stub no-summary needs to be issued on PSI. This tells the router that this area will not receive summary routes/ inter-area routes.

If we a take a look at OMEGA's routing table (before the change) you will notice that the inter-area routes and the default route generated by R2.

And if you isse the show ip ospf database command on PSI you can see what LSAs are in the database;

So if we take a look at the changes that have been made, we can see that OMEGA is no longer recieving the inter-area routes and PSI has changed the summary net link states for area 23.

Not So Stubby Areas

NSSAs are similar to regular stub areas, except that they allow routes to be redistributed from an ASBR into an area with a special LSA type, which gets converted to a normal external route at the ABR.

I will now change area 23 into an NSSA and generate an external route into the NSSA. This adds loopback 20 into OSPF which I have just removed;

If we look at PSI we can see that area 23 is now an NSSA and its performing LSA type 7 to 5 translation.

If we check the routing table on PSI we can see that the external route comes in as a type N2 from OMEGA. This is because it is a special NSSA external route.

If we look at CHIs routing table we can see that the route is a regular E2 external route, as PSI has performed the type 7 to 5 translation.

OMEGAs routing table also no longer has a default route and has inter-area routes.


We can change this by making the area a totally not so stubby area. So when we check OMEGAs routing table again, the inter-area routes have been replaced by a single default route.

If we take a look at PSI database we can see the various different LSA types.

OSPF Interface Authentication

To set up MD5 authentication between the serial links use the following commands;




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