Solved: OSPF Area ID

BRUNO WOLLMANN · ‎07-12-2005

Hello,

Does OSPF allow the use of duplicate area IDs? I've included a sample topology where area 0 is connected to 4 other areas. Can these 4 areas all have the same area id or should I label them 1 to 4? I've tested duplicate ID's in a lab and everything seems to be fine and I don't see any errors when debugging is turned on.

Any thoughts or comments?

thanks

Bruno

Richard Burts · ‎07-12-2005

Bruno

As far as I know you could implement OSPF with one area 0 and four instances of area 1 as your drawing represents. From a technical perspective it would work. But I would want to think for a minute not only about COULD you do this but SHOULD you do this? Are there reasons why you want to do this (other than the technical challenge of seeing whether it works)? Balanced against any possible reasons to implement this consider the possibility of confusion in the configuration and implications for supporting the network (especially if this is a live network rather than a lab).

So if you want to do this, it should work ok for you.

HTH

Rick

HTH

Rick

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ruwhite · ‎07-12-2005

The Area ID is not carried in type 3, type 5, or type 4 LSAs, so there's no restriction on using the same area ID for multiple flooding domains. I've seen a few designs where people use this trick for dial backup, so their dial link is in the same area as their primary link, even though they are using a centralized dial server (or a set of them). I know it's harder to troubleshoot when you do this, just from the perspective of: "I've telnetted to this router, and it's in the same area as the router I was just on, but it's not in the same flooding domain at all...."

:-)

Russ.W

View solution in original post

Richard Burts · ‎07-12-2005

Bruno

As far as I know you could implement OSPF with one area 0 and four instances of area 1 as your drawing represents. From a technical perspective it would work. But I would want to think for a minute not only about COULD you do this but SHOULD you do this? Are there reasons why you want to do this (other than the technical challenge of seeing whether it works)? Balanced against any possible reasons to implement this consider the possibility of confusion in the configuration and implications for supporting the network (especially if this is a live network rather than a lab).

So if you want to do this, it should work ok for you.

HTH

Rick

HTH

Rick

BRUNO WOLLMANN · ‎07-12-2005

Hi Rick,

I haven't made up my mind as to whether I should do this or not. I had to first find out if I could. The only reason I tested this in a lab was because I thought it would keep the configs on all my routers almost identical. I have no technical reason to do this. I only wanted to simplifiy config management.

Thanks for your response

Bruno

ruwhite · ‎07-12-2005

The Area ID is not carried in type 3, type 5, or type 4 LSAs, so there's no restriction on using the same area ID for multiple flooding domains. I've seen a few designs where people use this trick for dial backup, so their dial link is in the same area as their primary link, even though they are using a centralized dial server (or a set of them). I know it's harder to troubleshoot when you do this, just from the perspective of: "I've telnetted to this router, and it's in the same area as the router I was just on, but it's not in the same flooding domain at all...."

:-)

Russ.W