OSPFv2 DR and BDR Elections

Learning Objectives#

• Explain the necessity and operational role of DR (Designated Router) and BDR (Backup Designated Router) on multi-access broadcast OSPF networks.
• Examine default OSPF election criteria based on priority, manual router ID, and highest active interface IP address.
• Configure custom OSPF interface priorities using the ip ospf priority interface command to deterministically designate DR and BDR routers.
• Prevent a multi-access router from ever participating in elections by configuring priority 0 (DROTHER state).
• Verify OSPF adjacency states (FULL/DR, FULL/BDR, FULL/DROTHER) and timer intervals across multi-access broadcast segments.
• Demonstrate deterministic DR failover when the primary DR becomes unavailable.
• Verify the non-preemptive nature of OSPF elections when higher priority routers rejoin an established network.

Prerequisites#

• Thorough understanding of IPv4 subnetting and CIDR notation (/24).
• Familiarity with basic OSPF single-area operations, Link-State Advertisements (LSAs), and shortest-path-first (SPF) tree calculations.
• Experience navigating Cisco IOS CLI privileged EXEC and global configuration modes.

Topology#

The topology models an enterprise distribution segment connected through a central Catalyst 2960 switch (S1). Three Cisco 4321 routers (R1, R2, R3) connect their GigabitEthernet0/0/0 interfaces to S1 on the 192.168.1.0/24 multi-access broadcast subnet. An endpoint test station (PC-1) is connected to S1 on port FastEthernet0/4 to verify end-to-end data plane reachability across all advertised loopback subnets.

                  +-------------------+
                  |   Catalyst 2960   |
                  |     Switch S1     |
                  +---------+---------+
         Fa0/1         Fa0/2|          |Fa0/3        Fa0/4
           |                |          |               |
           |                |          |               |
     Gi0/0/0|          Gi0/0/0|     Gi0/0/0|            Fa0|
     +-----+----+     +-----+----+   +-----+----+   +------+------+
     |    R1    |     |    R2    |   |    R3    |   |    PC-1     |
     | DR (255) |     | BDR(100) |   |DROTHER(0)|   |192.168.1.50 |
     +----------+     +----------+   +----------+   +-------------+
       Lo0: 10.1.1.1    Lo0: 10.2.2.1  Lo0: 10.3.3.1

Addressing Table#

Lab Tasks#

1. Verify Baseline Multi-Access Layer 2 & 3 Connectivity:
   • Confirm all router interfaces on 192.168.1.0/24 are operational and pingable.
   • Confirm PC-1 can ping its default gateway 192.168.1.1.
2. Inspect Default OSPF Neighbors and DR/BDR Adjacencies:
   • Review initial OSPF neighbor tables and observe election outcomes before manual priority tuning.
3. Configure Deterministic OSPF Interface Priorities:
   • Configure ip ospf priority 255 on R1's GigabitEthernet0/0/0 interface to enforce DR status.
   • Configure ip ospf priority 100 on R2's GigabitEthernet0/0/0 interface to enforce BDR status.
   • Configure ip ospf priority 0 on R3's GigabitEthernet0/0/0 interface to disqualify it from DR/BDR elections.
4. Trigger OSPF Election Convergence:
   • Reset the OSPF process across routers using clear ip ospf process to force immediate re-election based on updated priorities.
5. Verify Multi-Access OSPF Neighbors and Roles:
   • Verify show ip ospf neighbor and show ip ospf interface output across all routers.
   • Confirm R1 is DR, R2 is BDR, and R3 is DROTHER.
6. Verify OSPF Route Propagation and End-to-End Connectivity:
   • Confirm R1, R2, and R3 advertise their respective loopback networks (10.1.1.0/24, 10.2.2.0/24, 10.3.3.0/24) into Area 0.
   • Verify PC-1 can reach all router loopback IPs.
7. Conduct DR Failover and Non-Preemption Negative Tests:
   • Administratively shut down R1's multi-access interface and verify that R2 promotes to DR while R3 remains DROTHER.
   • Re-enable R1's interface and verify that non-preemption prevents R1 from usurping the DR role until the next election event.

Configuration Walkthrough#

Step 1: Configure R1 Interface Priority and OSPF Settings#

R1(config)# interface GigabitEthernet0/0/0
R1(config-if)# ip ospf priority 255
R1(config-if)# exit

Step 2: Configure R2 Interface Priority and OSPF Settings#

R2(config)# interface GigabitEthernet0/0/0
R2(config-if)# ip ospf priority 100
R2(config-if)# exit

Step 3: Configure R3 Interface Priority to Disqualify from Election#

R3(config)# interface GigabitEthernet0/0/0
R3(config-if)# ip ospf priority 0
R3(config-if)# exit

Step 4: Reset OSPF Process to Force Re-Election#

Because OSPF elections are non-preemptive, priority changes do not immediately replace the active DR or BDR. Clear the OSPF process on all participating routers to force a new election:

R1# clear ip ospf process
Reset ALL OSPF processes? [no]: yes

R2# clear ip ospf process
Reset ALL OSPF processes? [no]: yes

R3# clear ip ospf process
Reset ALL OSPF processes? [no]: yes

Verification#

1. Multi-Access Neighbor Adjacencies on R1#

Verify that R1 has established full adjacencies with R2 (BDR) and R3 (DROTHER):

R1# show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
3.3.3.3           0   FULL/DROTHER    00:00:36    192.168.1.3     GigabitEthernet0/0/0
2.2.2.2         100   FULL/BDR        00:00:30    192.168.1.2     GigabitEthernet0/0/0

2. Interface OSPF Roles and Priorities#

Verify the assigned interface roles on each router:

R1# show ip ospf interface GigabitEthernet0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
  Internet address is 192.168.1.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 255
  Designated Router (ID) 1.1.1.1, Interface address 192.168.1.1
  Backup Designated Router (ID) 2.2.2.2, Interface address 192.168.1.2

R2# show ip ospf interface GigabitEthernet0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
  Internet address is 192.168.1.2/24, Area 0
  Process ID 1, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State BDR, Priority 100
  Designated Router (ID) 1.1.1.1, Interface address 192.168.1.1
  Backup Designated Router (ID) 2.2.2.2, Interface address 192.168.1.2

R3# show ip ospf interface GigabitEthernet0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
  Internet address is 192.168.1.3/24, Area 0
  Process ID 1, Router ID 3.3.3.3, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DROTHER, Priority 0
  Designated Router (ID) 1.1.1.1, Interface address 192.168.1.1
  Backup Designated Router (ID) 2.2.2.2, Interface address 192.168.1.2

3. Loopback Route Propagation#

Verify that R3 learns the loopback networks of R1 and R2 via OSPF:

R3# show ip route ospf
     10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
O       10.1.1.1 [110/2] via 192.168.1.1, 00:05:12, GigabitEthernet0/0/0
O       10.2.2.1 [110/2] via 192.168.1.2, 00:05:12, GigabitEthernet0/0/0

4. End-to-End Host Reachability#

From PC-1, verify ICMP reachability to all router loopback interfaces:

C:\> ping 10.1.1.1
Reply from 10.1.1.1: bytes=32 time=16ms TTL=255

C:\> ping 10.2.2.1
Reply from 10.2.2.1: bytes=32 time=1ms TTL=255

C:\> ping 10.3.3.1
Reply from 10.3.3.1: bytes=32 time=1ms TTL=255

Troubleshooting#

• Symptom: OSPF Neighbor stuck in 2-WAY/DROTHER state.
  • Cause: On a multi-access broadcast network, DROTHER routers form full adjacencies only with the DR and BDR. Adjacencies between two DROTHER routers remain permanently in 2-WAY/DROTHER. This is expected OSPF design behavior to prevent $N \times (N-1) / 2$ adjacency explosion.
• Symptom: Changed ip ospf priority but router is not elected DR.
  • Cause: OSPF elections are non-preemptive. If a DR and BDR already exist when a router boots or priority is raised, the incumbent DR remains active. Clear the OSPF process (clear ip ospf process) on all segment routers to force a new election.
• Symptom: Router with priority 0 was elected BDR.
  • Cause: Interface priority 0 definitively disables DR/BDR eligibility. Verify show ip ospf interface <int> to ensure Priority 0 is applied to the correct interface and area.

Key Takeaways#

• DR/BDR Scalability: On multi-access networks (Ethernet), DR and BDR act as central points of contact for LSA flooding. Routers send LSAs to 224.0.0.6 (AllDRouters); the DR floods updates to 224.0.0.5 (AllSPFRouters).
• Interface Priority: The ip ospf priority <0-255> command controls election preference (default is 1). Highest priority wins; priority 0 excludes a router completely from becoming DR or BDR.
• Tie-Breaking Mechanism: When interface priorities are equal, the router with the highest Router ID (configured RID, highest loopback IP, or highest physical IP) wins.
• Non-Preemptive Elections: Once elected, a DR or BDR remains in that role until it fails or its OSPF process restarts, ensuring network stability.

References#

• Cisco Systems: OSPF Neighbor States and DR/BDR Election Overview (cisco.com)
• Cisco IOS IP Routing: OSPF Configuration Guide, Cisco IOS Release 15M&T (cisco.com)
• Cisco CCNA 200-301 Official Cert Guide: OSPFv2 Concepts and Implementation

Files and Validation Status#