DR and BDR help please....

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Hello everybody.
I have a question.
On a single area or on every area using the OSPF routing protocol, the election of a DR and a BDR is as follow: highest IP address on an interface that is part of the OSPF routing process gets elected as the DR, the second highest IP address on an interface that is part of the OSPF routing process gets elected as the BDR, correct? This can override by the address of the loopback interface but same rule applies correct?

What IP address do I use for the loopback interface? Is it one available address within the same network?

On an area (OSPF) can we have two routers elected as BDR???

Or we have one DR and one BDR peer network?

If you guys see the picture, packet tracer is giving me two routers as BDR.

Please, any advice is grateful as I am just about to undertake the exam.

Cheers

 

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Albert,

Elections for DR/BDR don't occur on all ospf interfaces - DR/BDR are elected on broadcast multi-access networks. For example, there will be no election on the serial segments in your example network. There will be only one DR and one BDR on each segment; note that in your sample network R0 and R1 are on different segments. DR/BDR is by interface, so a router could be the DR on one segment and BDR or DROTHER on other segments.

Selection of the DR/BDR is based on highest priority first (a configurable value), next by highest router-id. The router-id can be configured (and doesn't need to be an actual ip address on the router); if it is not configured the router-id defaults to the highest loopback, if no loopback then the highest interface ip address.

A very important point - ospf does not preempt DR's/BDR's - so if a new router joins a segment with a higher priority, the existing DR will still be DR.

Loopbacks are typically assigned a /32 address in a seperate range. E.g. in your diagram perhaps give the routers loopbacks of 10.128.64.1 /32, 10.128.64.2 /32, etc.

Spice_Weasel

 

Yeah, correct.

Yes, that is why I left the link off. On a p2p we can only have 2 routers.

Exactly. So accordingly not only to what I have learnt from the book, you are saying what I would say. In the picture, router R0 is correct, it has the second lowest IP address hence BDR. Router R3 is the DR as it has the highest IP address for that segment but R3 is also the BDR for the other segment as it has the second highest IP address, correct? If so, why is R1 elected by packet tracer the BDR for that segment if it has the highest IP address? This is where I am lost here.

Through the CCNA book I haven’t read a bit about DROTHER, only about DR and BDR. What is DROTHER?

Correct, I have left that value as default so I have 1 on all the routers.

Once again, here we have, highest IP address. I do not have setup any loopback interfaces, so why is it that packet tracer is telling me that R1 is a BDR if it has the highest IP address?

Unless we reload all of them, correct?

Didn’t know that and the CCNA book makes no mention about, it does in fact shows us an IP from within the same network range as the router is. Our tutor was saying that we’d have to setup loopback address of 172.16.0.0 in all of them but I was like, hold on!!! If we give it all loopback addresses the same IP address how will it make its decision if the IPs are all the same? Unless we’d set up the priority on them.

Once again, invaluable info mate as the CCNA book makes no mention what so ever about this so naturally we where clueless.

I hope that I haven’t confused this even more than already is but I thing it is quite straight forward. If priority value is set to 1 by default in all routers and no loopback interfaces where created then it will use the interfaces IP address to make the decision on which will be DR and BDR. Routers with a priority set to 0 will no be part of the election.
Any more advice more then welcome.
Cheers mate.

 

Forgot to say that given all that info, then R3 should be DR for the 10.10.3.0 network and BDR for the 10.10.2.0 network.
No?

 

Almost - According to your diagram R0 and R1 should be DR's on their segments and R3 the BDR, assuming the priorities are same on each interface, and no router-id or loopback interfaces are configured. Although there are training materials that say router-id is determined by highest ospf interface (when no loopback interface is configured), in practice the router will select the highest interface regardless if ospf is running on it or not.

Again, to reiterate, DR is selected by:
-highest priority
-highest router-id

Router-id is determined by:
-configured router-id value
-highest loopback ip address
-highest interface ip address


Keep in mind that in real ospf networks routers will have loopback interfaces and often a router-id configured, as well as priority, to ensure control over which router is selected DR and BDR. It is a bit annoying that since there is no preemption you sometimes need to clear the ospf process in order to make the correct router the DR, or set the priority of other routers to 0 to prevent them from becoming DR/BDR.

Spice_Weasel

 

So, R0 is suposed to be the DR for it's own segment even it's IP address is lower (10.10.3.1) then the interface's IP address on router R3 which is higher (10.10.3.2)?
I am confused. Based on the few information seen on the diagram what routers are supose to be DR and BDR and why?

Mate I must say that you seem to know what you are talking about. I appreciate all your help as I need to get my head round this as I want to get this exam out of the way but I also want to get the subject right.

Cheers mate.

 

Take it step by step and you'll see which router should be DR. Consider the R0 - R3 segment:

Let's say both routers have a priority of 1, so DR should be selected based on router-id.
R0 has two interfaces, 10.10.3.1 and 10.10.4.2, so it will use 10.10.4.2 for the router-id, even if ospf is not configured for interface s0/0. R3 has two interfaces, 10.10.3.2 and 10.10.2.1, so it will select 10.10.3.2 as router-id. R0 has the higher router-id and therefore should be DR.

However, the bit I said about no preemption is important - if R3 was already the DR, then when R0 starts up a forms a neighbour relationship with R3, it will not take over as DR, but will become BDR. Sorting out the DR can sometimes be a bit tricky because depending on the order the ospf processes started on the various routers, a different router than expected could end up the DR.

Moral of the story: Define an ospf router-id - you don't want the router picking it! And use priority to limit which routers can become DR/BDR.

DROTHER is essentially what a router that is neither a DR or BDR will be. For example, on an ethernet segment you might see the neighbour state as FULL/DR, FULL/BDR or 2-WAY/DROTHER once the neigher relationships are established.

Spice_Weasel

 

Spice_Weasel, so I was correct in a way about my diagram, the truth is I didn't explained well and packet tracer is correct also.

I did not had set any IPs on the Serial Links and they were all down, basicly not set at all.

Once I did that, it was like you have said. The router R0 did use the IP address from the serial interface and used it as it's ID.

Now I got it.

Thanks mate, you have been a great help.
Very greatfull.
You take care.

 

Glad to help - good questions (like yours) benefit both of us, as I've always thought that you don't really understand something until you can explain it to someone else.

Good luck on your exam!

Spice_Weasel

 

Spice_Weasel I second the thanks from albertc30, and albertc30 cheers for raising the question in the first place,

I think i got this whole ospf thing sorted in my head (not as well nailed down as u Spice_Weasel) but my icnd2 exam is wednesday, and scenarios like this are worth their weight in gold!

Especially liked the real world tip "Define an ospf router-id - you don't want the router picking it! And use priority to limit which routers can become DR/BDR." Also the tip about being careful about what router started up first(or started the ospf process first) as making a change to who is the DR will have no effect till a re-boot or clear ip ospf has reset the process ...

Fantastic people .... gotta get more stuff like this up here

P.S
Spice_Weasel .. a question we are all wondering...how do you remember all this stuff? i know you obviously work hard for what you have, but dam - u da man! - an inspiration to the rest of us of what can be achieved.

 

I don't know SW personally... but I'd guess the answer is the same as mine: doing it.

I repped SW in the thread. Props!

 

Thanks everyone, I appreciate the compliments - certforums has a great community of people! AS BM said, using a technology day to day makes it easier to remember how it works. In my case I am fortunate to be the senior network engineer at small IT company, so almost all the interesting networking tasks and projects land on my desk. This means I get to do all the BGP, IGP's, layer 2, multicast, firewalls, all the fun stuff! Also, I am studying for my CCIE lab, which involves playing around to try and really understand how things work. That is one reason questions like Albert's can be such a good thing - it makes me stop and think about something I take for granted, and next thing you know I'm powering up some 2611's to test something, and probably noticing some little thing I hadn't seen before.

Spice_Weasel

 

Spice_Weasel, in your opinion what would be between these two labs the ideal lab for my CCNA lab, bear in mind that I will carry on to CCNP after I have done my CCNA.
Cheers mate.
Certforums are the best...

CCNA Lab 1

CCNA Lab 2

 

Hello everyone.
Just to ask you some questions. Please see the network diagrams and have your say.
The one with the setting a default route is tricky and I am a bit lost as it mentions IP address 172.16.6.6 as a default next hop on the gateway but I got it wrong as I was like, I have no IP on the other end of the network, in the cloud, so I went for the interface option although the option correct with all the options that propagate the default route to all other routers automatically was option number 3.

Anyways it was a pass with 91.3% now onto the next chapter, the EIGRP.
Hope it's as fun as the OSPF was.
Really enjoyed it.

Take care.

 

Hello

Well the question regarding propagating the default route in my working outs is option 3, the commands all seem correct and the next hop ip address - 172.16.6.6 is the end point of the P2P link,

If you check the wild card you notice it is 0.0.0.3 = 255.255.255.252 or /30

So working this back to binary to find the incriment of the host range; (this is how i work it out, not how you have too)

252 converted back into binary = 11111100,

Match this up with our decimal table 128,64,32,16,8,4,2,1, you notice that the network portion ends on the 4, this is our incremient.

sooo we use 4 as our incriment:
172.16.6.0 - 172.16.6.3 (usable .1 & .2)
172.16.6.4 - 172.16.6.7 (usable .5 & .6) <-- the IP assined to S0 falls within this range
172.16.6.8 - 172.16.6.11

On the diagram 172.16.6.5 has already been assigned to S0 on Router B, therefore the end of the P2P link has to be 172.16.6.6 - that make sense.

The other question regarding DR/BDR elections i believe is;
DR is R2 (as the election process looks for the highest priority on that given segment first)
BDR is R1 (as this ties with R3 on priority, so its now looking at highest IP on any interface at router startup, so R1 wins at loopback over-rides highest IP on a physical interface)

Hope that helps ..and is right information

 

Yeap, so I did thought that too, that any loop address would override any other physical interface's IP address but as you can see from the picture the answer is Router 3 with the IP on the s0 201.1.1.1/30.
Wonder why I lost some points on the d*** thing.

Anyways, cheers for the reply mate.

 

here is the diagram sorry...

 

albertc30,

Please remove the copyrighted test question images from your post immediately.

 

Sorry, Done it without even realizing it.