OSPF Area Design Question

[Filip Knezevic]

Hi all,

My question is about usage of NSSA areas and standard areas on ABRs.

So, say we have a 7606 router with multiple line cards. Each of the ports can lead to a transport switch serving many customers, with multiple routers behind. Obviously that is many interfaces, with a lot of routes.

My question is about the area design. Let's say 7606 has a backbone area and NSSA areas. We use NSSA areas behind transport switches, so that the end routers don't end up with a lot of routes.

However, Cisco design guides don't recommend more than 3 areas per ABR, but also about 50 routers in an area.

If we have around 20 OSPF adjacencies on the 7606, should they all be a part of one NSSA area or each transport switch should be a NSSA area for itself? Obviously you can't see routes from NSSA area 1.0.1.1 from NSSA area 1.0.2.1. So in theory, end routers will have less routes in their routing table if we use NSSA per transport switch, or per adjacency. Or we should stick to one NSSA area for all transport switches on 7606 and use OSPF filters to reduce the number of routes?

Another question is concerning the connected interfaces. Currently we have redistribute connected, which is bad as we have hundreds of interfaces on our chassis routers, thus NSSA areas are much larger than they should, as they have all those E2 routes. My plan is to assign all connected interfaces to a new standard area with network commands. that should help reduce the NSSA routing tables.

Your thoughts please.

Thank you.

[Giuseppe Larosa · Accepted solution]

Hello Filip,

>> However, Cisco design guides don't recommend more than 3 areas per ABR, but also about 50 routers in an area.

These numbers are quite old and related to when router processors were not so powerful.

Here, in the forums the use of a single area with 800 routers have been reported (by Russ White if I remember correctly).

So the reason to use multiple areas is actually to achieve better control on route propagation.

But ABRs can perform granular control (per prefix) only on internal routes (using area range or other commands to filter routes).

So your ideas are in the right direction.

First of all, you should convert all the redistributed connected into network commands in order to be able to summarize at area border.

About the use of more areas in a single device, you can think of using the backbone area and 10 totally NSSA.

I think to overcome single node failures you should have two core nodes or a VSS.

And I would put in the same NSSA area two distribution layer devices bulding also L3 links between them.

A few key points to remember about NSSA areas:

NSSA allows you to inject external routes into the area as LSA type 7 that can be converted to standard LSA type 5 on ABR.

All LSA type 5 from backbone are blocked not passed to the NSSA area (there is no reverse translation from type 5 to type 7 in ABR)

Last but most important one is for NSSA you need to configure default-route injection into the NSSA on ABR, this is not done by default for NSSA but only for stub areas.

Totally NSSA wil minimize the database of each NSSA area (you need the no-summary option on the ABR nodes) and provides the greatest scalability.

If you convert connected routes to internal routes you can use then area range commands in ABR nodes to reduce the number of routes injected into the backbone.

The use of NSSA areas will also provide a migration path for conversion of connected routes into network statements as NSSA supports redstributiom.

Hope to help

Giuseppe

[SamanBayat4424]

Hello

First of all, I recommended to still have those NSSA areas. You didn't have to merge them and make a big area. 

Second, why you want to change redistributed route with network command? Are you going to summarize Routes? Do you want to filter them? What ever you want to do, you could use redistributed with tags!

When you using tag option, then you can easily manage all redistributed Routes.

[Filip Knezevic]

Hi Giuseppe and Saman,

Thanks for your replies.

@giuseppe Thanks for confirming I'm on a good track. Looks like OSPF documentation can be a bit outdated with our ever progressing technology. 

@Saman,

I want to move redistribute connected interfaces to another area. Why? Only one of our many 7600 access routers has more than 600 redistributed interfaces. If I leave them to be redistributed, unfortunately the will be injected into all NSSA areas on that same router. So our NSSA areas currently have a lot of E2 routes, defying the whole purpose of NSSA as a separated zone. So a big part of the problem is the number of E2 routes. If I move all connected interfaces to a standard area, I will free a lot of routes from NSSA areas attached to the router.

Yes, I will proceed with an NSSA area per transport switch. And standard for connected interfaces.