OSPF Special Area Types!!

The topic which I have chosen for today is special area types in OSPF. I have seen that people ((I was one of them ;)) find it hard to grasp these area types.

We know that OSPF is Link State Interior Gateway Protocol which works by advertising Link State Advertisements (LSA) to its neighbours. LSA are nothing but state of router Interface. More neighbors in Autonomous System means more LSA you need to share with your neighbours and more processing in terms of Power, Memory, CPU is needed on routers to process those incoming LSAs.

What is the solution?

To divide the whole autonomous system into different Areas with Area 0 or Back bone area being at the centre or you can say all other areas connects to Area 0.

What is the Benefit?

With division of whole autonomous system into different Areas, routers have to send the LSAs only to its neighbors inside that particular area and not with all OSPF routers of autonomous system.

Then how the information flows outside the Area?

This is with the help of OSPF Area Border Routers (ABRs) which summarizes the LSAs from One area and send it to another area in Type 3 LSA which is also called Summary LSA.

Till this point we have not introduced any special area types. Everything I have mentioned till now is mostly about Area 0 (Backbone Area) and any other area which is connected to Area 0. Let’s say that another area as Area 1 and common point among both of these areas is ABR which is at the border between these 2 areas.

Before learning Special area types you have to understand one type of LSA which is called “External LSAs or Type 5 LSAs”. These LSAs advertises external connectivity. External connectivity could be from some other Autonomous system or if redistribution is happening from any other IGP, Static protocol into OSPF.

Now, Special Areas are listed as:

1) Stub Area

2) Totally Stubby Area

3) Not-so-Stubby Area

4) Totally Not-so Stubby Area

:S… What is this all about?    Ok, I will try to explain in simple terms  😀

As I discussed above that we have divided the Autonomous systems into Areas to restrict the flow of LSAs however there can be situations that you have some router in your network which is of very low memory or very old legacy router which can’t take all the routes in its routing table but instead of replacing it you want to keep it in your network and serve customers via it. You don’t want to bombard that router will extern LSAs to reach those external prefixes instead you can configure that router as Stub router.

With Stub router configuration all External LSA gets suppressed. But then how that router reaches External prefixes. That is via Default route. As soon as you configure Stub ABR router and other Internal routers as stub router, ABR will automatically advertises a default route towards Internal stub routers which is only information Internal stub routers needs in order to reach External prefixes.

Now in Stub routing, routers will still have Type 3 Summary LSAs, Type 1 Router LSA and default route in their database however why you even want Type 3 Summary LSA when you have default route. This is achieved via configuring the router as Totally Stubby Area where summary LSA even will be suppressed.

People think that apart from Stub area, all other area types are Cisco proprietary however if you look at the original OSPF RFC, Not so Stubby Area has been defined over there so it’s not exactly a Cisco Proprietary feature and has been implemented by other Vendors. Totally Stub Area and Totally Not-so Stubby Area are not defined in RFC.

There are many instances in which companies want to connect to another company or 2 companies gets merged and they want to share any information with each other but in limited fashion. This can be achieved by NSSA (Not-So-Stuby Area) in which router which connects to other’s company network becomes NSSA ASBR (Autonomous System Boundary Router) and router which is connected to Backbone Area 0 Router becomes NSSA ABR.

Redistribution into an NSSA area creates a special type of link-state advertisement (LSA) known as Type 7 LSA, which can only exist in an NSSA area. An NSSA autonomous system boundary router (ASBR) generates this LSA and an NSSA area border router (ABR) translates it into a type 5 LSA, which gets propagated into the OSPF domain i.e Area 0. However one thing to note is that External routes i.e. Type 5 LSAs coming via NSSA ABR is still not allowed into NSSA as normal Stub area rules still apply.

Like for Stub area we have Totally Stubby Area, for NSSA we have Totally NSSA which is same as before that NSSA doesn’t allow Type 5 LSA however it allow Type 3 Summary LSA’s , with Totally NSSA, we are suppressing this summary LSA even.. 🙂

Note: When you configure an area as NSSA, by default the NSSA ABR does not generate a default summary route. In the case of a stub area or an NSSA totally stub area, the NSSA ABR does generate a default summary route.

I know text has become lengthy but I didn’t want to stop abruptly to add to more confusion 🙂 . If you have any doubts, please let me know.

R

Mohit

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