OSPF DR selection question

Answered Question
Nov 11th, 2008

Hi,

I understand in multi-access environment if set "ip ospf priority 0", the interface is not able to participate in DR/BDR selection process which has a value 1 as default. Now consider the following scenario in a broadcast environment:

1. RouteA

interface fast0/0

ip ospf priority 255

2. RouterB

router ospf 1

router-id 192.168.254.1

3. RouterC

interface loopback0

ip address 222.222.222.222 255.255.255.255

My question is :-

a) which from the above will be selected as DR and BDR?

b) Does setting "ip ospf priority 255" supercede other rules in DR/BDR selection process ? (e.g. highest ip/ router-id)

I have this problem too.
0 votes
Correct Answer by mheusing about 8 years 4 weeks ago

Hi,

To reinforce what Jon wrote:

a) The router who comes up first will be DR and the second one BDR. DR/BDR election in OSPF is non-preemptive, so who got the job keeps the job.

IF an election process takes place RouterA will be DR and router C will be BDR.

b) Yes, if a DR/BDR election takes place

So in brief:

1) If there is no existing DR on a LAN and no other OSPF router as seen from the Hellos, then the router declares itself as DR unless the priority is set to 0.

2a) If there is already a DR and a BDR no change will occur, independent of priority. DR and BDR will keep their role, even if they have a smaller priority.

2b) If there is no BDR but a DR then the router will become BDR.

2C) If there is no DR and no BDR and Hellos from several routers are seen (all with 0.0.0.0 in the DR and BDR field) then election takes place. The routers must be able to form an adjacency (matching area, authentication, password, stub bit, etc. ..).

3) A priority of 0 means no DR or BDR role.

Regards,

Martin

Correct Answer by mohindersingh about 8 years 4 weeks ago

Hi Buddy,

if you set the OSPf priority of a router to 255 it will supercede all other information and will be elected as a DR and if any other router will also have a 255 priotity then router-id will be used as a decision maker.

it is always better to fix the router-id manually because in future if you configure a new loopback address with higher number than current router-id, so after reboot router will pick the new interface ip as router-id and whole OSPF database will be converged with new router-id.

Regarding interfaces, loopback interfaces preferred over physical interfaces in router-id selection.

Please rate the post, if find useful.

Mahi

Correct Answer by Jon Marshall about 8 years 4 weeks ago

Iqbal

OSPF priority will take precedence over the highest IP/router-id. Only if the priority is the same will the highest IP/router-id then be used to break the tie.

Note that if an OSPF router boots up and it has the highest priority but there is already and DR/BDR then it will not become either the DR or BDR.

Jon

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Correct Answer
Jon Marshall Tue, 11/11/2008 - 03:55

Iqbal

OSPF priority will take precedence over the highest IP/router-id. Only if the priority is the same will the highest IP/router-id then be used to break the tie.

Note that if an OSPF router boots up and it has the highest priority but there is already and DR/BDR then it will not become either the DR or BDR.

Jon

Correct Answer
mohindersingh Tue, 11/11/2008 - 04:31

Hi Buddy,

if you set the OSPf priority of a router to 255 it will supercede all other information and will be elected as a DR and if any other router will also have a 255 priotity then router-id will be used as a decision maker.

it is always better to fix the router-id manually because in future if you configure a new loopback address with higher number than current router-id, so after reboot router will pick the new interface ip as router-id and whole OSPF database will be converged with new router-id.

Regarding interfaces, loopback interfaces preferred over physical interfaces in router-id selection.

Please rate the post, if find useful.

Mahi

Correct Answer
mheusing Tue, 11/11/2008 - 05:28

Hi,

To reinforce what Jon wrote:

a) The router who comes up first will be DR and the second one BDR. DR/BDR election in OSPF is non-preemptive, so who got the job keeps the job.

IF an election process takes place RouterA will be DR and router C will be BDR.

b) Yes, if a DR/BDR election takes place

So in brief:

1) If there is no existing DR on a LAN and no other OSPF router as seen from the Hellos, then the router declares itself as DR unless the priority is set to 0.

2a) If there is already a DR and a BDR no change will occur, independent of priority. DR and BDR will keep their role, even if they have a smaller priority.

2b) If there is no BDR but a DR then the router will become BDR.

2C) If there is no DR and no BDR and Hellos from several routers are seen (all with 0.0.0.0 in the DR and BDR field) then election takes place. The routers must be able to form an adjacency (matching area, authentication, password, stub bit, etc. ..).

3) A priority of 0 means no DR or BDR role.

Regards,

Martin

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