OSPF DR/BDR election

[Sam-CCNP]

Hi,

I have been doing packet captures and debugs (debug ip ospf adj and lsa-generation) on a 3-router OSPF lab in CML and noticed that DR/BDR elections are taking place several times during the formation of the new adjacency between the (existing) DR and BDR, and the new DROTHER that comes up.

The first election is after 2-WAY is established, which makes sense as per the interface state machine.

But then approximately 10 seconds after the adjacency goes to Full, another two elections take place on both the DR and BDR  and I can't figure out why. The DROTHER only has a single election take place immediately after 2-WAY is established, and that is it - which is what I would expect of all three routers. 

I have re-run the lab several times, and the result is always the same (logs are below).

*May 25 12:58:30.568: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on GigabitEthernet0/0 from LOADING to FULL, Loading Done
<omitted>
*May 25 12:58:39.348: OSPF-1 ADJ Gi0/0: Neighbor change event
*May 25 12:58:39.348: OSPF-1 ADJ Gi0/0: DR/BDR election
*May 25 12:58:39.348: OSPF-1 ADJ Gi0/0: Elect BDR 1.1.1.1
*May 25 12:58:39.348: OSPF-1 ADJ Gi0/0: Elect DR 2.2.2.2
*May 25 12:58:39.349: OSPF-1 ADJ Gi0/0: DR: 2.2.2.2 (Id)
*May 25 12:58:39.349: OSPF-1 ADJ Gi0/0: BDR: 1.1.1.1 (Id)
*May 25 12:58:39.349: OSPF-1 LSGEN: Scheduling rtr LSA for area 0, build flag 0x41 (from 0x30484D5)
*May 25 12:58:39.349: OSPF-1 ADJ Gi0/0: Neighbor change event
*May 25 12:58:39.349: OSPF-1 ADJ Gi0/0: DR/BDR election
*May 25 12:58:39.349: OSPF-1 ADJ Gi0/0: Elect BDR 1.1.1.1
*May 25 12:58:39.349: OSPF-1 ADJ Gi0/0: Elect DR 2.2.2.2
*May 25 12:58:39.350: OSPF-1 ADJ Gi0/0: DR: 2.2.2.2 (Id)
*May 25 12:58:39.350: OSPF-1 ADJ Gi0/0: BDR: 1.1.1.1 (Id)
*May 25 12:58:39.350: OSPF-1 LSGEN: Scheduling rtr LSA for area 0, build flag 0x41 (from 0x30484D5)

The only reason I can see that this might occur - based on RFC2328, 9.2 Events causing interface state change - is when:

NeighborChange
o Bidirectional communication has been established to a neighbor. In other words, the state of the neighbor has transitioned to 2-Way or higher.

But that already occurred once.

Any help explaining this behavior would be appreciated.

Thanks,

Sam

[MHM Cisco World]

that I'm seeing additional elections on R1 (BDR) and R2 (DR)<<- you see elect after full in both or in only one ? make double check this point 

[Sam-CCNP]

Hi,

On both, I checked and can reproduce with the same result whenever I wish. 

Thanks.

Sam