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OSPF

suchit

Hi,

I have one question on ospf adjacency and DR, BDR elections.

When a new router is configured in ospf network, and there is already DR and BDR are elected in that network.And this new router doesn't have direct physical connection to DR and BDR then how this new router forms adjacency with the DR and BDR.

Any reply is greately appreciated.

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Comments

cisco_trooper

suchit wrote:

Hi,

I have one question on ospf adjacency and DR, BDR elections.

When a new router is configured in ospf network, and there is already DR and BDR are elected in that network.And this new router doesn't have direct physical connection to DR and BDR then how this new router forms adjacency with the DR and BDR.

Any reply is greately appreciated.

DRs and BDRs are only elected on shared media, such as ethernet. These devices are not technically directly connected. If you have 2 devices connected via ethernet and they have already been elected DR and BDR. The third will form an adjacency with both of them if it is properly configured, but the DR and BDR will remain as is unless you either reboot one of them, or you clear the ospf process. (yes a failure would cause this too)

tech-airman

suchit wrote:

Hi,

I have one question on ospf adjacency and DR, BDR elections.

When a new router is configured in ospf network, and there is already DR and BDR are elected in that network.And this new router doesn't have direct physical connection to DR and BDR then how this new router forms adjacency with the DR and BDR.

Any reply is greately appreciated.

suchit,

Let's assume the following simple network..

[RouterA]{FA0/0}----(Area 0)---{FA0/0}[RouterB]{FA0/1}----(Area 1)---{FA0/0][RouterC]

Since you mentioned "...a new router..." and "...this new router doesn't have direct physical connection to DR and BDR..." then let's assume RouterC is the "new router."

What you have to understand for Multi-area OSPF is that the router interface belongs to the network associated with an area. So instead of thinking "RouterA belongs to Area 0," instead think "interface fa0/0 of RouterA belongs to Area 0." Now, continuing that logic, "interface fa0/0 of RouterB also belongs to Area 0." So far, we've got two interfaces for Area 0. That means those two interfaces are involved in the DR and BDR elections for Area 0.

Now, just like interface fa0/0 of RouterA and interface fa0/0 of RouterB are part of the DR and BDR election for Area 0, interface fa0/1 of RouterB and interface fa0/0 of RouterC are involved in a SEPARATE DR and BDR election for Area 1. So to answer your question, the "...new router..." of RouterC does in fact create an adjacency with a DR and BDR for Area 1 but NOT with the DR and BDR of Area 0.

I hope this helps.

suchit

Thanks a lot

nice343

DR and BDR are also the elected on an NBMA network (frame-relay). But with an NBMA the neighbors have to be statically defined

suchit

Hi ,

[RouterA]{FA0/0}

{FA0/0}[RouterB]{FA0/1}

{FA0/0][RouterC] {FA0/1}

{FA0/0}[RouterD]

In this newly joined router is Router D and all the routers are in the same area ,if Router A is DR and Router B is BDR then how the Router D forms adjacency with the DR and BDR because it has no direct physical connection to DR and BDR.

Thanks

dtlokee

You need to go back and reread the concepts of the DR and BDR. There will be a DR and a BDR for EACH multiaccess network, not per area. In the example you provided each FastEthernet interface is a seperate multiaccess network and therefore will have a seperate DR and BDR. In the example you described each router will be a DR/BDR for each one of the connected interfaces because there are only 2 devices connected to the data link.