OSPF DR query

tinu_karki

Hi all,

I had always assumed that it was only possible to have one DR in a single Area 0. If there were two routers with the same priority then the DR would get chosen on the basis of the highest mac. However tonight when doing a practise lab in GNS3 I am seeing two DR's in Area 0? Am i missing something here? The network is a standard broadcast network and routers Topaz and Quartz both have interfaces that are DR's on area 0. Attached is the topology and output is below:


On router Topaz you can see that interface Fa1/0 is the DR for Area 0:
Topaz#sh ip ospf int br
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Fa1/0 100 0 192.168.12.2/24 1 DR 1/1
Fa0/0 100 1 192.168.23.2/24 1 DR 1/1
Topaz#sh ip ospf int fa1/0
FastEthernet1/0 is up, line protocol is up
Internet Address 192.168.12.2/24, Area 0
Process ID 100, Router ID 192.168.23.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 192.168.23.2, Interface address 192.168.12.2
Backup Designated router (ID) 192.168.14.1, Interface address 192.168.12.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:07
Supports Link-local Signaling (LLS)
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 3, maximum is 3
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 192.168.14.1 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
Topaz#sh run | b router ospf
router ospf 100
log-adjacency-changes
area 1 range 172.16.0.0 255.255.252.0
network 192.168.12.0 0.0.0.255 area 0
network 192.168.23.0 0.0.0.255 area 1
!


On router Quartz interface Fa0/0 is the DR for area 0:
Quartz#sh ip ospf int fa0/0
FastEthernet0/0 is up, line protocol is up
Internet Address 192.168.14.4/24, Area 0
Process ID 100, Router ID 192.168.45.4, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 192.168.45.4, Interface address 192.168.14.4
Backup Designated router (ID) 192.168.14.1, Interface address 192.168.14.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:01
Supports Link-local Signaling (LLS)
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 0, maximum is 3
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 192.168.14.1 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
Quartz#sh run | b router ospf
router ospf 100
log-adjacency-changes
area 2 range 10.10.0.0 255.255.248.0
network 192.168.14.0 0.0.0.255 area 0
network 192.168.45.0 0.0.0.255 area 2
!

fredrikjj

There's one DR per broadcast segment. It has nothing to do with areas.

Dieg0M

Uh how do you not know this?

braincork

"If there were two routers with the same priority then the DR would get chosen on the basis of the highest mac."

If by mac you mean router ID, loopback, or ip then this statement is correct.

powmia

And in reality, it's the router that comes online first... there is no preemption of the DR role. If the routers come online at the same time, yes... highest RID wins the election.

Every broadcast segment has a DR. The OP's topology has two individual ethernet links in area 0. Each routed ethernet link is considered it's own broadcast segment.

While you're labbing this up, go ahead and put the command "ip ospf net point-to-point" on each of those links and count your DRs afterwards Then think about why that's important.

tinu_karki

Dieg0M wrote: »

Uh how do you not know this?

Working with BGP and security for the last few years, havent looked at OSPF for a long time. just getting back into studying, thought this forum was about helping each other, not being patronising

Thanks to the rest for clarifying!

gorebrush

powmia wrote: »

And in reality, it's the router that comes online first... there is no preemption of the DR role. If the routers come online at the same time, yes... highest RID wins the election.

Every broadcast segment has a DR. The OP's topology has two individual ethernet links in area 0. Each routed ethernet link is considered it's own broadcast segment.

While you're labbing this up, go ahead and put the command "ip ospf net point-to-point" on each of those links and count your DRs afterwards Then think about why that's important.

Best answer in this thread, imo.

Scales

Maybe this will help also?
OSPF Design Guide - Cisco