OSPF Routing -Designator ID

[Cisconew]

Hi,

I have done OSPF lab. In that I have noticed OSPF Designated ID in every Ethernet link is confusing about selection.

The selection Process is based on Priority or Router ID.

"OSPF deems a router more preferable if the priority for the interface is the highest for that segment. If the OSPF priority is the same, the higher RID is more favorable."

I have verified this on link between R2 and R4.Here, Priority is

same(=1).Then,R2's Router ID is 0.0.51.2 and R4's Router ID is 0.0.51.4

So, my guess is it should be R4's

Router ID 0.0.51.4

But , the below output from R2 and R4 showing Designated

Router ID as 0.0.51.2

Can someone explain? 

R2#sh ip ospf int G 0/4
GigabitEthernet0/4 is up, line protocol is up
Internet Address 24.24.24.2/24, Area 51, Attached via Network Statement
Process ID 1, Router ID 0.0.51.2, Network Type BROADCAST, Cost: 1
Topology-MTID Cost Disabled Shutdown Topology Name
0 1 no no Base
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 0.0.51.2, Interface address 24.24.24.2
Backup Designated router (ID) 0.0.51.4, Interface address 24.24.24.4
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:02
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/3/3, flood queue length 0
Next 0x0(0)/0x0(0)/0x0(0)
Last flood scan length is 0, maximum is 4
Last flood scan time is 1 msec, maximum is 1 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 0.0.51.4 (Backup Designated Router)
Suppress hello for 0 neighbor(s)




R4#sh ip ospf int G 0/2
GigabitEthernet0/2 is up, line protocol is up
Internet Address 24.24.24.4/24, Area 51, Attached via Network Statement
Process ID 1, Router ID 0.0.51.4, Network Type BROADCAST, Cost: 1
Topology-MTID Cost Disabled Shutdown Topology Name
0 1 no no Base
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 0.0.51.2, Interface address 24.24.24.2
Backup Designated router (ID) 0.0.51.4, Interface address 24.24.24.4
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:08
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/2/2, flood queue length 0
Next 0x0(0)/0x0(0)/0x0(0)
Last flood scan length is 3, maximum is 4
Last flood scan time is 0 msec, maximum is 2 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 0.0.51.2 (Designated Router)
Suppress hello for 0 neighbor(s)

[MHM Cisco World]

The only explanation is you modify ospf  priority after DR election take place.

So to check 

Wr config 

Power off router and then power on and check again.

The highest router ID must be elect 

[David Ruess · Accepted solution]

Yes, its quite possible R2 came up and didn't see any other router on the link and elected itself a DR...then R4 came online and sent its messages but since the DR/BDR process isn't premptive (mostly) then R4 just allowed itslef to be the BDR.

I would try to bring up both routers at the same time as close as you can to see if it makes the correct decision.

-David

[Cisconew]

It has been changed, after I issue

clear ip ospf process

in R2.

Thanks.

[Giuseppe Larosa]

Hello @Cisconew ,

from the

show ip ospf

interface there is the line related to the timers:

>> Timer intervals configured,

Hello 10, Dead 40, Wait 40, Retransmit 5

the wait timer is the time a router waits for other neighbors before electing himself to DR.

The wait timer defaults to 40 seconds.

So what likely happened is that you enabled OSPF on R2 first and on R4 more then 40 seconds later and as explained by @David Ruess  once a DR exists in a segment a new router cannot

pre-empt

By using

clear ospf

process on R2 you made possible for R4 to take part in DR election and to win it.

Hope to help

Giuseppe

[Richard Burts]

Most of the explanations of DR/BDR elections assume an already running network where in an election the router with the highest priority wins. David provides a helpful explanation that at startup the processing is different. Which router came up first is more important than which router has highest priority. And in the already running network the

clear ip ospf process

command initiates a normal election.