From: Godswill Oletu (oletu@inbox.lv)
Date: Thu Jun 01 2006 - 15:34:23 ART
Jian,
The concept of preemption does not apply in a two router OSPF segment. When
one router goes down, so does the OSPF ajacency and neigbor relationship with
the other router. The second router cannot transistion from a BDR to a DR all
by itself, neither will it remain a BDR in the absent of the other router. So,
when the old router comes back online, there will be no DR or BDR on that
segment, the OSPF ajacancy will be renegotiated from the beginning as if it
never occurred before and so will be the election of DR/BDR.
HTH
Godswill Oletu
----- Original Message -----
From: Jian Gu
To: Godswill Oletu
Cc: Cisco certification
Sent: Thursday, June 01, 2006 1:25 PM
Subject: Re: DR Election: Incase of a Priority Tie, Highest RID Wins - Truth
or Fallacy?
DR does not preempt.
On 6/1/06, Godswill Oletu <oletu@inbox.lv> wrote:
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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