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New Member

OSPF IP addressing

I have been reading a lot about OSPF design and tuning and I have been unable to find anything about IP addressing between routers in an area. OSPF elects a DR and a BDR for every broadcast domain so you can end up having multiple DR/BDR's in the same area. If you have all links between routers in the same broadcast domain, then there will only be one DR and BDR for that particular area; is that prefered? All routers still have to perform the SPF algorithm but the DR/BDR send out the LSA's to intra-area routers. Would all the routers being in the one broadcast domain save a little bit of bandwidth or processing power?

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1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

It is a common

It is a common misunderstanding to believe that an area should have a single DR and a single BDR. But that is not the case. In fact DR and BDR have nothing to do with areas.

 

I think the situation that you describe might be like this

RouterA has an Ethernet connection to RouterB

RouterA has a separate Ethernet connection to RouterC

RouterA has a separate Ethernet connection to RouterD

RouterA has a separate Ethernet connection to RouterE

So RouterA is a member of 4 separate broadcast domains. And yes RouterA would be DR or BDR in each of the 4 broadcast domains (assuming that there are not other routers connected to any of these broadcast domains). It is not necessarily an issue for RouterA to be DR or BDR in multiple networks).

 

HTH

 

Rick

3 REPLIES
Hall of Fame Super Silver

Perhaps there are some things

Perhaps there are some things that you intend in your question that I am not understanding. You are correct in understanding that OSPF elects a DR and a BDR for each broadcast domain (which is essentially saying for each network/subnet) in the area. So your question asks about having all links between routers in the same broadcast domain, which means they are all in the same subnet. While it is certainly possible to have all the routers in an area to be in the same subnet that is an unusual design. It is difficult to say whether that is preferred because it is so unusual.

 

HTH

 

Rick

New Member

I understand. For some reason

I understand. For some reason I thought that each area was only suppose to have one DR/BDR and the only way to accomplish that would to be have all the links between the routers in the same subnet. Would it be typical to just have a /30 or /31 on each link in the backbone? If one router connects to four other routers (all seperate broadcast domains), wouldn't that router either be a DR or BDR for four different broadcast domains? I think I might need to make a picture to explain it easier at this point lol.

Hall of Fame Super Silver

It is a common

It is a common misunderstanding to believe that an area should have a single DR and a single BDR. But that is not the case. In fact DR and BDR have nothing to do with areas.

 

I think the situation that you describe might be like this

RouterA has an Ethernet connection to RouterB

RouterA has a separate Ethernet connection to RouterC

RouterA has a separate Ethernet connection to RouterD

RouterA has a separate Ethernet connection to RouterE

So RouterA is a member of 4 separate broadcast domains. And yes RouterA would be DR or BDR in each of the 4 broadcast domains (assuming that there are not other routers connected to any of these broadcast domains). It is not necessarily an issue for RouterA to be DR or BDR in multiple networks).

 

HTH

 

Rick

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