[c-nsp] Newbish OSPF DR question on VLANs

[Brett Frankenberger]

On Thu, Jun 07, 2007 at 12:57:39PM +0300, Hank Nussbacher wrote:
> At 05:00 AM 07-06-07 -0400, Rodney Dunn wrote:
> >Also depends if you have the priority set and who came up first.
> 
>  From the OSPF Design Guide:
> http://www.cisco.com/warp/customer/104/1.html
> 
> "DR and BDR election is done via the Hello protocol. Hello packets are
> exchanged via IP multicast packets (Appendix B) on each segment. The
> router with the highest OSPF priority on a segment will become the DR
> for that segment. The same process is repeated for the BDR. In case of a
> tie, the router with the highest RID will win. The default for the
> interface OSPF priority is one. Remember that the DR and BDR concepts
> are per multiaccess segment. Setting the ospf priority on an interface
> is done using the ip ospf priority <value> interface command."
>
>  [ ... ]
> 
> There is something I am missing here that I am trying to understand.

DR election is not pre-emptive.  Once one router becomes the DR, it
will remain the DR as long as it remains up; another, higher
priority/router-id router coming up won't replace it.

When two routers come up at roughly the same time, the router with the
highet priority will become DR, with ties broken by router-id, kist as
you describe above.  (Similarly, if a subnet splits long enough for a
router on both sides to become DR, and then the subnet is rejoined,
you'll have two DRs and the one with the highest priority or router-id
will win that contention and remain the DR.)

If a lower-priority/router-id router comes up first and is up long
enough to declare itself the DR, another router coming up later won't
replace it; similarly, if the highest priority/router-id router is the
DR and it goes down, a lower priority/router-id router will become the
DR and will remain DR even after the higher priority/router-id router
comes back up.

Consequently, over the long haul, what you generally end up with is
that the DR is the router with the longest uptime.  

An exception is that if you configure a router with priority 0,
it will never become DR.  (So, if all the routers on the subnet are
priority 0, then there will be no DR for that subnet.)

     -- Brett