OSPF: DR election

[vpalania]

Hi,

In RFC2328 section 9.4(Electing the Designated Router),I have the following doubts:

1)In the second point,'Only those routers on the list that have not declared themselves to be Designated Router are eligible to become Backup Designated Router'.In this statement, what does it mean 'declared themselves' ?Is it sending Hello packets with DR as its own Router ID ?

2)In the fourth point,'If Router X is now newly the Designated Router or newly the Backup Designated Router, or is now no longer the Designated Router or no longer the Backup Designated Router, repeat steps 2 and 3, and then proceed to step 5'.Why is it needed to repeat step 2, after electing DR ?

Thanks,

Vijay

[mheusinger]

Hi,

A1) assume a router with no neighbors on a LAN. It will go through the steps in section 9.4 and finally find itself in the position of the DR. In case another router is joining the same LAN it will find the already existing DR as this one will send hellos with DR as its own router ID.

A2) from RFC2328 section 9.4 (4)

"For example, if Router X is now the Designated Router, when step 2 is repeated X will no longer be eligible for Backup Designated Router election. Among other things, this will ensure that no router will declare itself both Backup Designated Router and Designated Router."

The main idea is that DR and BDR election are non-preemptive. This means if a router is DR/BDR it will remain DR/BDR when a new router joins the LAN. 9.4 (4) just ensures that the DR will be another router than the BDR once there are at least two elegible routers in the LAN.

Hope this helps

Martin

[vpalania]

Hi,

I am not clear wiht your second answer.Once the DR elected, there are only the following cases:

1)BDR is already elected. or

2)No DR, hence BDR is promoted to DR.

Hence, after this election is over.Why is it needed to repeat step 2 to check again ?

Thanks,

Vijay

[mheusinger]

Hi,

I do not agree to your statement. When you boot a router with a LAN interface and there is no other router it will become DR. And there will be no BDR, because 9.4(4) prevents the same router to elect itself as BDR.

When a DR election takes place, there can be the following situations:

1) no DR and no BDR (f.e. LAN switch boots and all routers get link up at the same time), router X is elegible

2) no DR and no BDR (f.e. LAN switch boots and all routers get link up at the same time), router X is not elegible

3) DR failed, router X is BDR

4) DR failed, router X is not BDR but elegible (prio not 0)

5) DR failed, router X is not elegible (prio=0)

In all cases router X needs to follow the same algorithm and in all cases a new DR and potentially a new BDR should be elected. Once a BDR "is promoted" to be DR the BDR election will take place.

Again the check is needed to ensure a router would not become DR AND BDR preventing a second router to become BDR.

Hope this helps

Martin

[vpalania]

Hi,

Thanks for your reply.

But I am not cleared with your reply.Could someone explain in other way.

Thanks,

Vijay

[pkhatri · Accepted solution]

Hi Vijay,

I presume you are happy with the answer to your first question and just need clarification on your second question.

Here's an example of where this applies.

Consider a segment where only one router, RTA, is eligible to become a DR. When it first comes up,it will run the DR election process. Here is what happens as per the steps in section 9.4 of RFC2328 (the numbers correspond to the corresponding step numbers in section 9.4).

1. There is no current DR or BDR.

2. Since no other routers will have declared themselves BDR, RTA will consider itself the BDR for now.

3. Since no other routers will have declared themselves DR, RTA will assign the Designated Router to be the same as the newly elected Backup Designated Router, which happens to be itself. So at this point, RTA considers itself both a DR and a BDR.

4. At this point, since RTA is now newly both the DR and the BDR, it has to repeat steps 2 and 3.

2 - Repeat. This step says:"Only those routers on the list that have not declared themselves to be Designated Router are eligible to become Backup Designated Router". Since RTA considers itself to be the DR, it is no longer eligible to be the BDR as per the previous statement. Therefore, since there are no other DR-eligible routers on the segment, RTA will decide that the segment has no BDR>

3 - Repeat. RTA has to choose the DR from among those routers that have declared themselves DR. In this case, it's only RTA so it will now declare itself DR.

So here's what the repetition of steps 2 and 3 do.

- at the end of the first iteration, RTA considers itself both DR and BDR

- at the end of the second iteration, RTA considers itself just the DR and there is no BDR on the segment.

The second iteration was done to ensure that the same router (i.e. RTA) does not consider itself both DR and BDR.

Hope that makes it a bit clearer.

Paresh.

[vpalania]

Hi,

Thanks for your reply.

Vijay