From: Petr Lapukhov (petrsoft@gmail.com)
Date: Thu Jun 01 2006 - 14:24:18 ART
Godswill,
The main idea for DR election with OSPF is that it is
"non-preemptive", i.e not deterministic, as it is
with IS-IS.
Actually, when router come alive on a shared network, it first
spend some time "waiting" for DR/BDR announces (WAIT time).
That is, if it hears DR/BDR announce in neighbors' hello packets,
it will not even bother participating in new elections, even if it has
higher priority or RID.
So if you really want some router to ALWAY be a DR, you should
set all other routers' priorities to zero.
That thing is explained in details in RFC2328 :)
HTH
Petr
2006/6/1, Godswill Oletu <oletu@inbox.lv>:
>
> Hi,
>
> This topic was beaten to death the past few weeks on the group and the
> general
> concession is that, when there is a tie on the priority vlaues, the
> highest
> Router-ID wins, Cisco online documentation have various pages confirming
> this
> as well. But, I do not know if anyone labbed this up and fool-proof this
> concept.
>
> I am have labbed that exact scenario, that required that a particular
> router
> be elected the DR in segment of two routers, the neighbor or priority
> commands
> are not to be used.
>
> The results I am getting is not consistent across the board:
>
> Little preview of my configures:
>
> Rack1R2:
> interface Serial0
> no ip address
> encapsulation frame-relay
> no frame-relay inverse-arp
> !
> interface Serial0.204 point-to-point
> ip address 144.1.24.2 255.255.255.0
> ip ospf network broadcast
> frame-relay interface-dlci 204
> !
> router ospf 1
> router-id 222.2.2.2
> log-adjacency-changes
> network 144.1.24.2 0.0.0.0 area 0.0.0.0
> network 150.1.2.2 0.0.0.0 area 0.0.0.0
> !
>
> -----------------------------------------------------------------------------
> --
> Rack1R4:
> interface Serial0/0
> no ip address
> encapsulation frame-relay
> no frame-relay inverse-arp
> !
> interface Serial0/0.402 point-to-point
> ip address 144.1.24.4 255.255.255.0
> ip ospf network broadcast
> frame-relay interface-dlci 402
> !
> router ospf 1
> router-id 150.1.4.4
> log-adjacency-changes
> network 144.1.24.4 0.0.0.0 area 0.0.0.0
> network 150.1.4.4 0.0.0.0 area 0.0.0.0
> !
>
> -----------------------------------------------------------------------------
> -------
> Results:
> Rack1R2#clear ip ospf process
> Rack1R2#sho ip ospf nei
> Neighbor ID Pri State Dead Time Address
> Interface
> 150.1.4.4 1 FULL/DR 00:00:37 144.1.24.4
> Serial0.204
> Rack1R2#
>
> Rack1R4#show ip ospf nei
> Neighbor ID Pri State Dead Time Address
> Interface
> 222.2.2.2 1 FULL/BDR 00:00:32 144.1.24.2
> Serial0/0.402
> Rack1R4#
>
> Despite the fact that Rack1R4 have the lowest Router-ID, it was elected
> the DR
> for that segment and Rack1R2 who have the highest Router-ID settled for
> the
> less fancy job of a BDR.
>
> Now............
>
> Rack1R4#clear ip osp nei
> Rack1R4#sho ip ospf nei
> Neighbor ID Pri State Dead Time Address
> Interface
> 222.2.2.2 1 FULL/DR 00:00:39 144.1.24.2
> Serial0/0.402
> Rack1R4#
>
> Rack1R2#sh ip ospf nei
> Neighbor ID Pri State Dead Time Address
> Interface
> 150.1.4.4 1 FULL/BDR 00:00:34 144.1.24.4
> Serial0.204
> Rack1R2#
>
>
> Now, the roles have been revised, completely negativing the 'supposed'
> influence that a higher Router-ID should have in the DR/BDR election
> process.
>
> Or, are mine missing something here? Maybe my coffee have not sink in
> yet...but your contribution is highly welcome.
>
> Thanks.
> Godswill Oletu
>
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