Saturday, November 6, 2010


Virtual Links do not belong in a normal network, but serves as a band-aid to temporarily fix network problems introduced when companies acquire other companies. Virtual Links allows areas to be connected to an area other than Area 0. A typical Virtual Link scenario looks like this:

In this topology, R1 has the Router ID set to, R2 is and R3 is The Router ID is very important in Virtual Link because you'll have to define it in the command. In this topology, we'll have to establish a Virtual Link through Area 1.

We'll first go to the router that connects to the backbone in the area. That would be R2. On R2, we'll use the following command:
area 1 virtual-link

The command would be the same regardless of how many routers are between R2 and R3. Now, we'll do the same command on R3:
area 1 virtual-link

You simply have to remember that the area always specifies the area that you're going through.

A Virtual Link is a tunnel to trick the process to believe that it is really connected to the backbone area. You can check the virtual link using:
show ip ospf virtual

There are many ways this can fail, and most commonly it's a mistyped Router ID, a changed Router ID, or wrong Area ID.

If the situation presents itself, you can actually do Virtual Links within Virtual Links!

Now I'm going to go into different area types. This is the third time I'm using this image! But oh well, here goes!

Notice that in the image, I have Area 1 written as a possible Stub and Area 2 written as a possible Not-So-Stubby-Area.

Now, notice that no matter what external routes Area 1 receives, it only has one exit out of the network. It doesn't need to know about all the other routes and ways out of the network because it makes no difference!

In a Stubby area, all Type 5 LSAs are blocked. Recall that Type 5 LSAs are external routes sent in from ASBRs. So if an Area is Stubby, it has all the routes in the OSPF domain, and a default route to the ABR.

Now, to step it up further, you can configure an area as a Totally Stubby area. In a Totally Stubby Area, Type 3, 4 and 5 LSAs are blocked. And if we recall, Type 3 is an ABR summary and Type 4 and 5 refers to the ASBR's IP and ASBR External Routes respectively. A default route to the ABR is also flooded.

To configure an area as a Stub, you simply go on the ABR and all Internal routers and type:
area 1 stub

The ABR sees no change in the routing table, but if we hop on to an Internal router, we would see a new default Inter-Area route pointing to the ABR.

Totally Stubby Areas are Cisco proprietary. The only equipment that needs to be Cisco is the ABR. The Internal routers can still use the "area 1 stub" command. The Totally Stubby Area command to be used on the ABR is:
area 1 stub no-summary

The "No-Summary" keyword refers to Summary LSAs. Remember that Type 3 and 4 are both called Summary LSAs. These are now blocked as well. Now, the routers in Area 1 should only know about local prefixes, and a default route towards the ABR.

Now, what if a router in a Totally Stubby area becomes an ASBR? You end up with a Not-So-Stubby-Area, and that's a real technical term! NSSAs passes external routes using a Type 7 LSA, which gets read as a Type 5 once they reach the ABR. You can have a Not-So-Stubby-Area, and a Not-So-Totally-Stubby-Area.

To change Area 2 into an NSSA, type this on the ABR:
area 1 nssa

Likewise, the "no-summary" keyword can be put behind to denote a Totally Stubby area. The ASBR in the NSSA would receive a default route to the ABR in addition to all the RIP routes it knows and redistributes.

When any internal router in the Not-So-Stubby-Area wants to access the external routes from its area's own ASBR, it will first go to the ABR, then back through itself to the ASBR then out.

Like Virtual Links, NSSAs are band-aids to fix design goof-ups. It is not standard practice to design networks with NSSAs in mind.

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