Multi-areas in OSPF routing questions

[tinhnho123]

Hi Guys,

I currently have 2 x L3 switches, one is for area 0 and the other one is for area 5 and they're connected to each other. Since there are only 2 VLANs on each switch, I'd like to replace both of them with a better switch but unsure about the areas routing on the same L3 switch. Please let me know what your thoughts are.

Switch A is connected to switch B thru interface g1/0/1 with ip address: 10.10.10.1/26

I'm currently using ospf and here is the configure of switch A:

   router ospf 1

       network 172.20.1.1 0.0.0.0 area 0

       network 172.21.1.1 0.0.0.0 area  0

       network 10.10.10.1 0.0.0.0 area 5

And this is OSPF routing configure of switch B:

    router ospf 1

       network 192.168.1.1 0.0.0.0 area 5

       network 10.10.10.2 0.0.0.0 area 5

I'd assume the new switch C is the replacement of switch A & B. And here is my routing configuration of switch C:

     router ospf 1

       network 172.20.1.1 0.0.0.0 area 0

       network 172.21.1.1 0.0.0.0 area 0

       network 10.10.10.1 0.0.0.0 area 0

      network 192.168.1.1 0.0.0.0 area 5

       network 10.10.10.2 0.0.0.0 area 5

Is this going to work and/or best cisco practices? if not please advise. Thanks.

[Reza Sharifi]

Hi,

Since you are replacing 2 switches with 1, now you don't need the transit subnet (10.10.10.1/26) between the 2 switches any more. What will the new switch connect to? Is the new switch going to peer with any other device? If it is a stand alone switch, why do you even need 2 OSPF areas or even need OSPF all together?

HTH

[tinhnho123]

The area 0 where all the production servers are. The area 5 is where non-production servers. The new switch will connect to servers in both areas. I see your point that we don't need transit subnet. But we do need to have separate areas for these servers. Any suggestions? Thanks.

[tinhnho123]

The other idea that I've been thinking is setting up 2 ospf routings for these 2 areas on the same switch:

router ospf 1

      network 172.20.1.1 0.0.0.0 area 0

       network 172.21.1.1 0.0.0.0 area 0

      network 192.168.1.1 0.0.0.0 area 5

router ospf 2

      network 192.168.1.1 0.0.0.0 area 5

       network 10.10.10.2 0.0.0.0 area 5

What do you guys think? and why? Thanks.

[Bilal Nawaz]

Having 2 separate routing instances can become complex, and I wouldn't advise to do this.

If you will have one switch, you can simply enable ospf on the interfaces in their respective area's.

It could be arguable, but if you have tens/hundreds of router's in your network, then its worth the hassle of splitting your area's. If you haven't then what's the point?

How much processing power are you going to be saving...

[tinhnho123]

Yes, i do have over 100 routers and layer 3 switches. There has been 2 areas for long time, I'm just replacing the switches and combine them into one switch. This project which I'm working on is just a small project. I agree there won't be much processing power that I'm going to save but i just wanted to make sure that I'm following the best practices.

[Bilal Nawaz]

Yes, in which case, you can make that switch in to ABR. If you have 2 ospf instances, you will require redistribution and then your switch will become ASBR.

So you are pretty much right....

router ospf 1
network 172.20.1.1 0.0.0.0 area 0
network 172.21.1.1 0.0.0.0 area 0
network 10.10.10.1 0.0.0.0 area 0 <--- will you be using this after migration on the new switch, if not it can be taken out
network 192.168.1.1 0.0.0.0 area 5
network 10.10.10.2 0.0.0.0 area 5

hth

Bilal