DR and BDR elections

Understand

The DR and BDR are elected per broadcast or NBMA segment, not per area. Priority 0 is ineligible; among eligible candidates higher interface priority wins, then higher router ID breaks ties. Existing DR/BDR roles are non-preemptive: a new high-priority router does not automatically replace them.

DROTHER routers become Full with the DR and BDR, and normally remain 2-Way with each other. The DR originates the segment’s Type 2 network-LSA. Its control-plane role does not make it a compulsory data-plane next hop. On broadcast links, DROTHER flooding uses AllDRouters 224.0.0.6; the DR floods toward AllSPFRouters 224.0.0.5.

Lab setup

Use a fresh lab for this lesson; changes from other lessons are not cumulative. All outputs are illustrative excerpts, not recorded router tests. Transit cost is explicitly 10 and Loopback0 cost is 1. Interface names may vary by image.

Loopback0 is 10.255.0.<router number>/32; area assignments: R1: area 0, R2: area 0, R3: area 0, R4: area 0.

configure terminal
hostname R1
router ospf 1
 router-id 10.255.0.1
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.0
 ip ospf network broadcast
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

configure terminal
hostname R2
router ospf 1
 router-id 10.255.0.2
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.0
 ip ospf network broadcast
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

configure terminal
hostname R3
router ospf 1
 router-id 10.255.0.3
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.3 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.3 255.255.255.0
 ip ospf network broadcast
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

configure terminal
hostname R4
router ospf 1
 router-id 10.255.0.4
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.4 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.4 255.255.255.0
 ip ospf network broadcast
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

Apply

For a deterministic fresh election, configure priorities before enabling OSPF on Gi1. Use the LAN baseline addressing, withhold ip ospf 1 area 0 on Gi1 until the priorities below are set, then enable each router. Let R1 and R2 elect roles before R3/R4 join.

R1

configure terminal
interface GigabitEthernet1
 ip ospf priority 100
 ip ospf 1 area 0
end

R2

configure terminal
interface GigabitEthernet1
 ip ospf priority 50
 ip ospf 1 area 0
end

R3 and R4 · independently

configure terminal
interface GigabitEthernet1
 ip ospf priority 0
 ip ospf 1 area 0
end

Verify and interpret

R3# show ip ospf neighbor
Neighbor ID     Pri State     Dead Time Address     Interface
10.255.0.1      100 FULL/DR   00:00:36  192.0.2.1  GigabitEthernet1
10.255.0.2       50 FULL/BDR  00:00:34  192.0.2.2  GigabitEthernet1
10.255.0.4        0 2WAY/DROTHER 00:00:38 192.0.2.4 GigabitEthernet1

R3 and R4 are DROTHERs. Their 2-Way relationship is expected, not an MTU failure.

Now shut Gi1 on R1. R2 promotes from BDR to DR; R3/R4 cannot become BDR because their priorities are zero. Restore R1: it does not preempt R2. Record the new roles. Avoid resetting an entire production process merely to change an election.

R1 · failure then restore

configure terminal
interface GigabitEthernet1
 shutdown
end

R1 · restore after observing R2

configure terminal
interface GigabitEthernet1
 no shutdown
end

R2# show ip ospf interface GigabitEthernet1 | include State
  Transmit Delay is 1 sec, State DR, Priority 50

R2 remains DR after R1 returns. Restore all routers to the fresh baseline before a new election experiment.

Technical references

OSPFv2 standard · Router configuration guide