OSPF DR/BDR and Master/Slave

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Jun 27th, 2009
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By setting priority to a higher value on the router that has lower Router ID, it is easy to have DR/BDR router become Slave during Exstart phase. Looking into RFC2328 and specifically the fact that Master is the only one who increments sequence numbers during DBD exchange, I am having difficulty to understand how will LSA exchange happen when DR/BDR are not Master. If I plug a new router into multiaccess segment and it will have higher Router ID, than DR on that segment (so it will be Master in that relationship), how will it be able to receive all the LSUs from the DR?



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Giuseppe Larosa Sat, 06/27/2009 - 10:37
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Hello David,

the Master decides the sequence numbers to be used in the database descriptor packets that are different from LSA sequence numbers.

if the new router is the Master on the Exstart phase:

it decides the initial sequence numbers

it is the first to sends out its own DDs listing the LSA headers it knows.

This ends in a single packet listing its own router LSA.(external routes could be locally generated too).

The DR acknowledges the DD and puts in its list of requests the new LSAs.

However, the slave / DR node has to be allowed to send its own DDs because its LSDB is bigger for sure.

probably this could be signalled with the M more bit set indicating that it has to tell something to the otherside.

I agree that RFC2328 is not clear in this part.

A test can easily be done using

debug ip ospf adjacency on both routers the complete exchange could be seen.

I mean in real world the exchange happens correctly.

To be noted that because the new router cannot preempt the DR role even if it has the highest priority often in real world the DR is the device with the highest uptime on the LAN segment.

Hope to help



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