OSPF DR/BDR question

[BeakedPoint]

I was looking at a question with a topology similar to this and it was asking about DR/BDR elections. The answer said that R3 would be the DR for the 10.5.0.0 network, which makes sense; but, R2 would be the DR for the 10.4.0.0 section. That is what I'm confused about since shouldn't R1 be the DR due to the higher IP? Or are routers not allowed to be a DR/BDR in two networks at the same time? i.e. R2 being a BDR in 10.4 and 10.5.

[David Ruess · Accepted solution]

Hello,

The election process is:

Highest interface priority (default is 1)

Router with Highest Router ID

Router with highest Loopback interface

Router with highest configured physical interface in the UP state

Based of your example there isn't enough information to determine a DR/BDR. However I would "assume" the RIDs are R1 - 1.1.1.1, R2 - 2.2.2.2, R3 - 3.3.3.3. That being the case R2 would be the DR for the segment between R1 and R2 as it has the higher RID, adn that would make R3 the Dr for its segment as you have said. Highest physical interface IP is the last choice so depending on other configuration the DR can be either router. That and it doesn't even have to be the physical interface of the segment its electing the DR/BDR on.

Also to answer your other question yes a router can be a DR for multiple segments as its determined per segment. 

-David

[BeakedPoint]

That makes sense to me IF those loopbacks are set, i.e. 1.1.1.1, etc. But I was only given the IPs of the interfaces and nothing more. So, it's just a bit confusing when the only option was that R2 is the DR between it and R1 since, like you say -- and I've seen elsewhere said -- the election is segment dependent.

Thanks.

[David Ruess]

Yeah there should be more info. and a correction to my fist statement. Technically there IS enough information to determine the DR if we are to assume a default priority, no loopbacks or RID manually set and that's the only interface on the routers. However, given the answer you got I'd say either the answer is wrong or there isn't enough info provided to get to the right answer.

[Joseph W. Doherty]

One other possible issue determining DR/BDR with information provided (or lack there of), switches are shown between routers.  This allows router interfaces to be enabled at different times.  As DR/BDR isn't preemptive, an existing DR will not be replaced if a "better" DR router comes on-line.  (Of course, so far there's only two routers per shared segment, but resolution of this question becomes even more difficult if a third router was to be added.  Oh, if only two routers were to be on a shared segment, change relationship to p2p, making this question moot.  BTW, the latter doesn't require p2p addressing.)

[mohamedlamine]

The election process for selecting a Designated Router (DR) and Backup Designated Router (BDR) in a OSPF network typically follows these steps:

1. When a new OSPF router first comes online, it becomes a candidate to become the DR or BDR. It initiates the election process by sending Hello packets to discover other routers in the network.

2. Each router in the network compares its Router Priority with that of other routers. The Router Priority is a value ranging from 0 to 255, with higher values indicating a higher likelihood of becoming the DR or BDR.

3. The router with the highest Router Priority is elected as the DR, and the router with the second highest Router Priority becomes the BDR. If multiple routers have the same highest priority, the router with the highest OSPF router ID is selected as the DR.

4. The DR and BDR are responsible for managing communication within the OSPF network. The DR maintains adjacencies with all other routers in the network, while the BDR takes over if the DR fails.

5. The election process is dynamic and can be triggered by changes in the network, such as a router going offline or the Router Priority of a router being modified.

Overall, the DR and BDR election process is essential for ensuring efficient communication and routing within an OSPF network. By selecting designated routers, the network can effectively manage traffic and maintain network stability.

[Joseph W. Doherty]

@mohamedlamine you may want to read this https://community.cisco.com/t5/routing/reselection-of-dr-ospf/td-p/4803100.  I.e. an OSPF router coming on-line in a DR/BDR segment doesn't necessarily trigger an DR/BDR election.

[mohamedlamine]

thank you @Joseph W. Doherty