OSPF Neighbors and Adjacencies

[Raha_1990]

Hi!could you explain how to do such activity and explain the answers?

thanks!

[cadet alain]

Hi,

1) criteria for router-id selection:

  1° hard-coding with OSP process command  router-id x.x.x.x

  2° highest up/up Loopack IP address

  3° highest up/up IP address of physical interface

as no router-id configured under OSPF process here then:

RTE: Lo0 IP address

same for A and D

C: E1 IP

I let you find for other routers

2) criteria for DR selection per multipoint interface:

1° highest interface priority: ip ospf priority xx interface command( default is 1)

2° if tie-break in 1° then use the router with highest router-id on the segment

Here everyone has same priority so criterion 2 will be chosen

So between E and D: E is DR and D is BDR

between A,B , D and C:Router A is DR and D is BDR, others are DROTHERS

for 10.1.19.0 router A is DR

for last subnet there is no DR as this is a point-to-point interface

Regards.

Alain

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[Raha_1990]

are you sure that between E and D in the 2nd question E is DR?

[cadet alain]

Hi,

yes as it has the highest RID.

Regards.

Alain

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[Richard Burts]

Alain has correctly described the process used in the election for DR and BDR when all routers are active and participate in the election. But the question of who is actually the DR at a particular point in time can get complex. If one router (with a lower RID) boots up and becomes active before the other router boots up, then the lower RID will become the DR. Or if there had been an election and the higher RID was the DR and then the DR interface goes down (or the router reboots, or some other event that causes the higher RID to become not active) then the lower RID will become the DR and will remain the DR when the higher RID joins the network.

HTH

Rick

[Raha_1990]

ok,but for the network 10.1.10.0 the answer is RTB,not RTA.how could you explain it?

[cadet alain]

Hi,

yes this is RTR B because it has the highest router-id:209.165.201.1. Sorry for the mistake, I answered too quickly and forgot to take the serial interface into account for RID.

Regards.

Alain

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[Raha_1990]

I'm sorry but the answer for the network 10.1.13.0 is RTD ,not RTE.but the problem concerning 10.1.10.0 network is solved

[cadet alain]

Hi,

yep correct the RID is derived from Loopback so highest RID is D, this is what happens when you want to type faster than your brain 

Regards.

Alain

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[Raha_1990]

ok.thank you