I am studying OSP now and I thought I understood the DR/BDR election until I got confronted with this question that really confused me



What will be the result of the DR and BDR elections for this single area OSPF network?

(choose 3)

1- HQ will be DR for 10.4.0.0/16.
2- Router A will be DR for 10.4.0.0/16.
3- HQ will be BDR for 10.4.0.0/16.
4- Router A will be DR for 10.5.0.0/16.
5- Remote will be DR for 10.5.0.0/16.
6- Remote will be BDR for 10.5.0.0/16.


can someone explain it to me better?

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In my opinion, based on the information given, if the routers have formed adjacencies, Router A would be the BDR for each subnet, and the attached router for the corresponding subnet would be the DR. I think the question is flawed.

In reality, which ever router were to be turned on first, or which ever router the OSPF process was enabled on first for the area, would be elected the DR and BDR since OSPF doesn't allow for preemption of DR/BDR status.

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As I recall, DRs and BDRs are elected per multicast segment, not per area. Therefore you will have two DR/BDR pairs: one on 10.4.0.0/16, another on 10.5.0.0/16.

You probably know that the criterion to elect the DR is the router's priority (highest wins); at equal priorities, highest router id wins. I think it's fair to assume that all routers have the same priority (otherwise they would've told you the priorities). It's also fair to assume that the router id on a router is its highest IP address (otherwise they would've told you the configured router ids). Thus the router ids are 10.4.0.2 for HQ, 10.5.0.1 for A and 10.5.0.2 for Remote.

With the information you have, you can see that:

- on 10.4.0.0/16, DR is HQ, BDR is A.
- on 10.5.0.0/16, DR is Remote, BDR is A.

smrkdown makes a good point about preemption, and he is right. [wrong argument removed.]

(There is also the case when router A is configured with priority 0, in which case it is not eligible, so you would have DR without BDR, but that's beyond the scope of your question.)

Last edited by dmaftei on 09-20-04 at 12:23 PM

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So dmaftei and I agree as to which routers will fulfill which roles. I do disagree that a router must have first formed a neighbor relationship before it will perform the DR/BDR election process. I verified this in my lab. I turned on only one router, and it did elect itself as the DR for its broadcast interface (eth0). It did not however elect itself as the BDR. No other routers were active on the internetwork.

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You don't have an "election" per se when only one router is present. In OSPF as in many things, when a a router first comes up it assumes it is the boss. There hasn't been an election, it is simply waiting to hear from others.

-JV

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And..... Since it is initially the boss, it will remain the boss since there are no preemption capabilities.

And yes it will go through the election process regardless if it is the only router present or not.

This is why it is important to use priorities or loopback addresses if you actually care which routers will be the DR and BDR.

But don't take my word for it. Take the Carl Timm and Wade Edwards word for it:

"to further the example, if initially there is only one OSPF router interface active on the segment, it becomes the DR. The next router would become the BDR. Subsequent routers would all accept the existing DR and BDR and form adjacencies with them"

And Andrew Colton:

"When an interface first becomes functional, it checks whether the network has a designated router. If there is one, the router accepts that designated router regardless of its own OSPF priority"

Or just try it on your own lab and see what happens.

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Last edited by smrkdown on 09-20-04 at 02:19 AM

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quote:

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Originally posted by smrkdown
Or just try it on your own lab and see what happens.

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I don't pride myself on being right. I actually had to test it to see who was right.

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quote:

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Originally posted by smrkdown
I don't pride myself on being right.

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Router A will actually be the DR on the 10.4.0.0/16 subnet. Router A's router-id will be 10.5.0.1 and the HQ routers will be 10.4.0.2. Routers running OSPF take their highest loopback address, if available, as their router-id. If there aren't any loopback then it takes the highest address of the other interfaces.

Zac

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