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Community Member

OSPF: Same Area ID

What happens if you have two ABR routers, each with the same area id on your network? For example,

routerA area-0 and area-1

routerB area-0 and area-1

I know it's not best practice. What problems would this cause? Thanks in advance.

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Bronze

Re: OSPF: Same Area ID

You can partition non-zero areas without problem within OSPF. The problem is only with area 0.

The design above should work w/o problems as both areas will traverse the Area 0 and they will see each other as "O IA" instead of "O"

Hall of Fame Super Gold

Re: OSPF: Same Area ID

David

I mostly agree with Edison that it makes little difference if you have 2 areas as area 1 and they are not connected. The one thing to be careful about which could be an issue: if the ABRs are configured to do summarization for area 1, then it could become ambiguous which area 1 a particular subnet belongs to. So it is possible that someone forwards a packet to routerA but the subnet destination is really reachable through routerB, but routerA would not know this.

HTH

Rick

8 REPLIES
Hall of Fame Super Bronze

Re: OSPF: Same Area ID

You can partition non-zero areas without problem within OSPF. The problem is only with area 0.

The design above should work w/o problems as both areas will traverse the Area 0 and they will see each other as "O IA" instead of "O"

Hall of Fame Super Gold

Re: OSPF: Same Area ID

David

I mostly agree with Edison that it makes little difference if you have 2 areas as area 1 and they are not connected. The one thing to be careful about which could be an issue: if the ABRs are configured to do summarization for area 1, then it could become ambiguous which area 1 a particular subnet belongs to. So it is possible that someone forwards a packet to routerA but the subnet destination is really reachable through routerB, but routerA would not know this.

HTH

Rick

Hall of Fame Super Bronze

Re: OSPF: Same Area ID

Router A's area 1 isn't Router B's area 1 so Router B can summarize area 1 without affecting Router A's area 1 so I don't understand the concern here.

Community Member

Re: OSPF: Same Area ID

I find this discussion educational.

Ediortiz, are you stating that this would not be a problem because the two routers representing the same areas (area 1) would, in this scenario, only summarize their respective area 1 subnets through the backbone area?

For example, Router A's area 1 would contain subnet 192.168.2.0/24 and Router B's area 1 would contain subnet 192.168.3.0/24. Therefore, summarizing these subnets into the backbone would cause no problems.

Is this your conclusion as well?

Hall of Fame Super Bronze

Re: OSPF: Same Area ID

That's correct.

Community Member

Re: OSPF: Same Area ID

The scenario that Rick highlighted is actually valid for any summarisation. It's not specific to the same area ospf setup mentioned in this thread.

HTH,

Andres

Hall of Fame Super Gold

Re: OSPF: Same Area ID

Andres

This is a good point. My response was based on remembering an incident I experienced where there was an OSPF area with 2 Border routers which were configured to summarize the addresses within the area. There were some problems which resulted in partition of the area - producing 2 different area 1 with overlapping address ranges. The summarization on the Border routers created problems until the area partition was repaired.

Your point is well taken that the problem applies to any 2 OSPF areas with overlapping address ranges and which are configured to do summarization. It is not specific to the condition of 2 areas with the same area ID.

HTH

Rick

Hall of Fame Super Silver

Re: OSPF: Same Area ID

Hello Rick,

>> producing 2 different area 1 with overlapping address ranges

this is the point, without this additional condition as Edison noted there is no drawback in the duplicated non-zero area.

Going into details:

the OSPF LSA type 3 summary route can contain summarized or not summarized IP prefixes but does not contain the information about the original area where the prefix is injected in the OSPF database.

Another point that I had misunderstood in my first studies is the scope of LSA type3:

they live in a single area.

So how they can be seen in multiple areas and be in some ways domain wide ?

the ABR of (area x,0) originates the IP prefixes within area x into area 0 but also regenerates all LSA type 3 learned in area 0 as Area x LSA type 3 if x is not a form of totally stub area.

an internal router in area X sees a prefix of area y with an LSA generated by OSPF ABR (0,x).

some years ago we had to join two networks both using the same non-zero area and it worked well.

Hope to help

Giuseppe

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