[c-nsp] Newbish OSPF DR question on VLANs

[Hank Nussbacher]

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."

No priority is set.

So how is it that on router A:
A#sho ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.1.4       1   FULL/DR         00:00:35    10.1.3.2        Vlan50
192.168.1.2       1   FULL/DR         00:00:37    10.1.1.2        Vlan20

sees 192.168.1.4 as DR and on router C:
C#sho ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.1.1       1   FULL/DR         00:00:37    10.1.3.1        Vlan50
192.168.1.3       1   FULL/DR         00:00:38    10.3.3.1        Vlan40
192.168.1.2       1   FULL/DR         00:00:30    10.2.2.2        Vlan30

on the same VLAN50 it sees 192.168.1.1 as DR?

See http://www.interall.co.il/vlans.png for simple network schematic.

There is something I am missing here that I am trying to understand.

Thanks,
Hank



>What is the uptime of C?
>
>On Thu, Jun 07, 2007 at 11:48:52AM +0300, Hank Nussbacher wrote:
> > What causes a router to become a Designated Router (DR) on a VLAN?  I
> > always thought highest router-id, but I have a few test routers and 
> here is
> > what I get:
> >
> > C#sho ip ospf nei
> >
> > Neighbor ID     Pri   State           Dead Time   Address         Interface
> > 192.168.1.1       1   FULL/DR         00:00:37    10.1.3.1        Vlan50
> > 192.168.1.3       1   FULL/DR         00:00:38    10.3.3.1        Vlan40
> > 192.168.1.2       1   FULL/DR         00:00:30    10.2.2.2        Vlan30
> > C#sho ip ospf neigh 192.168.1.1 ?
> > C#sho ip ospf neigh 192.168.1.1 deta
> >  Neighbor 192.168.1.1, interface address 10.1.3.1
> >     In the area 0 via interface Vlan50
> >     Neighbor priority is 1, State is FULL, 6 state changes
> >     DR is 10.1.3.1 BDR is 10.1.3.2
> >     Options is 0x52
> >     LLS Options is 0x1 (LR)
> >     Dead timer due in 00:00:34
> >     Neighbor is up for 00:27:35
> >     Index 1/1, retransmission queue length 0, number of retransmission 1
> >     First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
> >     Last retransmission scan length is 1, maximum is 1
> >     Last retransmission scan time is 0 msec, maximum is 0 msec
> > C#sho ip ospf neigh 192.168.1.2 deta
> >  Neighbor 192.168.1.2, interface address 10.2.2.2
> >     In the area 0 via interface Vlan30
> >     Neighbor priority is 1, State is FULL, 6 state changes
> >     DR is 10.2.2.2 BDR is 10.2.2.1
> >     Options is 0x52
> >     LLS Options is 0x1 (LR)
> >     Dead timer due in 00:00:31
> >     Neighbor is up for 00:27:49
> >     Index 3/3, retransmission queue length 0, number of retransmission 0
> >     First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
> >     Last retransmission scan length is 0, maximum is 0
> >     Last retransmission scan time is 0 msec, maximum is 0 msec
> > C#sho ip ospf neigh 192.168.1.3 deta
> >  Neighbor 192.168.1.3, interface address 10.3.3.1
> >     In the area 0 via interface Vlan40
> >     Neighbor priority is 1, State is FULL, 6 state changes
> >     DR is 10.3.3.1 BDR is 10.3.3.2
> >     Options is 0x52
> >     LLS Options is 0x1 (LR)
> >     Dead timer due in 00:00:37
> >     Neighbor is up for 00:28:02
> >     Index 2/2, retransmission queue length 0, number of retransmission 1
> >     First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
> >     Last retransmission scan length is 1, maximum is 1
> >     Last retransmission scan time is 0 msec, maximum is 0 msec
> >
> > IOS 12.3(11)T2 if it makes a difference.
> >
> > All four of them become DR.  I have attached a png file that I hope makes
> > it thru the Mailman filter.
> >
> > Thanks,
> > -Hank
>
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