OSPF DR election time

[Volodymyr Khrystenko]

Hello.

I think we all know how DR is elected. But there is no answer how much time is needed for a router to become the DR?

For example we have 4 routers in broadcast network, all are shut down. We start the first one, it starts OSPF and sends hello to the segment. And what now? Noone wiil answer, so router must become the DR but how much time it waits? On google there is no answer to this interesting question(((

[JohnTylerPearce]

I could be wrong, and I have a feeling Peter will come into this thread, he usually does on stuff like this.

If you have a broadcast network, obviously there will be a DR and BDR. When OSPF attempts to form an adjacency, part of this process is selecting a DR and BDR. I would think, that if you admin shut down all but one, then it will not receive any responses back, and therefore will not form an adjacency, so no DR will be elected. As well as there is a OSPF adjacency and an OSPF neighborship. Since the OSPF adjacency will not be formed, then no DR will get elected.

Hope this helps.

If you're worried about which one gets elected, you can always modify this, by using OSPF priority values.

[JohnTylerPearce]

Just a quick FYI

They're are several states that an OSPF router will transition through, to form an OSPF adjacency.

1. Down

2. Attempt

3. Init

4. 2-Way

5. ExStart

6. Exchange

7. Loading

8. Full

I'll list reasons for the first two, which should get the point across.

Down
----
Down is the starting state for all OSPF routers. A start even, such as configurign the protocol, transition the router to the init state. The local router may list a neighbor in this state when no hello packets ahve been received within the specified router dead interval for that interface.

Init
----
The Init state is reached when an OSPF router received a hello packet but the local RID is not listed in teh received neighbor field. This means that bidirectional communication has not been established between the peers

[Richard Burts]

John

Actually there will be a DR even when it is the only router active on that subnet. With Cisco OSPF when there is a broadcast segment with at least one router then there will be a DR.

It can be pretty interesting if you do this on a router that has an Ethernet interface to a segment where it will be the only router.

- Configure OSPF to run on that interface.

- verify that OSPF did start running and has this interface as an interface in this process.

- shut down the Ethernet interface.

- run debug for OSPF adjacency.

- no shut the interface.

The debug output will show that the router waits briefly (I think it may be 30 seconds but it has been quite a while and I am not sure of the exact time) and then it will elect itself as the DR.

HTH

Rick

[JohnTylerPearce]

Richard,

So, I suppose that as long as the interface is in up/up, that the router will elect itself the DR, if it's the only router on the segment at that point in time?

[Richard Burts]

John

Yes that is correct. The router will elect itself if it is the only one active on the subnet.

HTH

Rick

[Rolf Fischer · Accepted solution]

 but how much time it waits? On google there is no answer to this interesting question(((

Hi,

from RFC 2328:

Wait Timer

A single shot timer that causes the interface to exit the Waiting state, and as a consequence select a Designated Router  on the network.  The length of the timer is RouterDeadInterval seconds.

-----------------------------------------------------------

R1#show ip ospf interface vlan 11 | incl Timer

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

HTH

Rolf

[Volodymyr Khrystenko]

Hello to everyone.

I think I  found the answer. There is timer called Wait Timer (found in show ip  ospf interface). By default it is 40 seconds. So the answer 40 seconds -  time for DR election.