Alex,

[cc'ing the list as the answer may be interesting to ppl]

I have to correct my previous answer. RTA's timer expiration
does not matter as, in fact, RTA does not elect itself as
the DR or the BDR until RTB's timer expires. Below is
the dynamics of the process:

t1: RTA and RTB have the Wait timer running.
    Prios: RTA: 5 RTB: 10

t2: RTA's wait timer expires, triggering
    DR/BDR election with the following results:

    BDR election: RTB (higher prio)
     DR election: RTB (no rtr announces itself as DR,
                  so BDR is promoted)

    RTA does not run the second round of BDR/DR election
    as its local state hasn't changed, it's still DROther.

    RTA starts announcing it's view of the world in it's
    hellos, but this does not cause RTB to get out of the
    Waiting state as RTA is neither the BDR nor the DR.

t3: RTB's timer expires. RTB elects the BDR/DR pair
    as follows:

    BDR election: RTB
     DR election: RTB

    Since RTB has just become the DR, it reruns the election
    algo to make sure he has consistent results with other
    folks on the segment:

    BDR election: RTA (RTB elected itself as the DR, so
                  it's taken out of the BDR election)
     DR election: RTB

    RTB announces its results in the Hello packets,
    causing NeighborChange on RTA's interface FSM.

t4: RTA's FSM experiences NeighborChange that triggers
    DR/BDR election consistent with RTB:

    BDR election: RTA (RTB is out as the DR)
     DR election: RTB

Actually, this topic (delayed DR/BDR convergence in situations
where BackupSeen cannot be generated) has been discussed in
the following paper:
http://portal.acm.org/citation.cfm?id=166243&coll=portal&dl=ACM&CFID=761976&CFTOKEN=28498926
Bottom line is that the DR election process always completes within
a constant time.

--
Alex Zinin
Monday, November 19, 2001, 10:24:46 PM, alex lerin wrote:
> Alex,
>   Thanks for your reply.
>   But still one doubt. The router RTA say RTB as DR.
>   So RTB consider RTA as BDR and not itself as DR.
>   It promotes RTB from BDR to DR. SO the result is RTA
> says RTB as DR and RTB says RTA as DR.
>   Can u clear this?
> Thanks,
> Alex
> --- Alex Zinin <azininnexsi.com> wrote:
>>
>> Alex,
>>
>>  I might be missing something in your explanation.
>>
>>  When the Wait timer expires on RTA, it will elect
>> the DR/BDR
>>  and will start announcing them in it's hellos. When
>>  RTB receives RTA's hellos, it will generate
>> BackupSeen
>>  event for the interface FSM, which will trigger the
>> DR/BDR
>>  election process giving consistent results.
>>
>> --
>> Alex Zinin
>>
>> Monday, November 19, 2001, 12:56:52 AM, alex lerin
>> wrote:
>>
>> > Hi,
>>
>> > I hava a doubt in DR-BDR election.
>> > Consider two routers RTA and RTB in a broadcast
>> > network.
>> > RTB has higher priority than RTA.
>> > RTA comes up first, and before RTA's Wait Timer
>> period
>> > expires, RTB also comes up.
>> > During RTA's DR election, RTA sees RTB in state
>> 2-Way,
>> > and elects RTB as the DR (since it has higher
>> > priority), and RTA as BDR.
>> > Now, During RTB's DR election, it sees no DR in
>> the
>> > network, and RTB as BDR. So, it elects RTB as DR,
>> and
>> > RTA as BDR.
>> > The condition is that each router is calling the
>> other
>> > DR and itself as BDR.
>> > Is there any way to avoid this miscalculation?
>>
>> > thanks
>> > Alex
>>
>>
>>
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