OSPF Designated Router (DR) and Backup Designated Router (BDR)

Open Shortest Path First (OSPF) is a link-state routing protocol designed for efficiency in large networks. On multi-access networks like Ethernet, OSPF uses Designated Routers (DRs) and Backup Designated Routers (BDRs) to optimize traffic flow and reduce overhead. These roles centralize adjacency formation and Link-State Advertisement (LSA) exchanges, minimizing unnecessary bandwidth consumption and CPU load.

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Key Points

• DRs and BDRs reduce network traffic by limiting full adjacencies to a hub-and-spoke model.
• Elections are based on OSPF interface priority (highest wins) and Router ID (tie-breaker).
• Once elected, a DR is non-preemptive—it won’t be replaced unless it fails or OSPF resets.
• The BDR takes over if the DR fails, and a new BDR is elected.
• All routers maintain an identical Link-State Database (LSDB); the DR only optimizes flooding.

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Why OSPF Uses a Designated Router

On broadcast or multi-access networks, OSPF avoids a full-mesh adjacency model (where every router forms a relationship with every other router). Without a DR:

• Each router exchanges LSAs with all others, increasing bandwidth usage.
• CPU load spikes due to excessive synchronization.
• Network convergence slows as the number of routers grows.

The DR introduces a hub-and-spoke model: Non-DR routers form full adjacencies primarily with the DR (and BDR), reducing overhead while maintaining a distributed topology.

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How DRs and BDRs Work

Core Functions

1. Centralized Adjacency Formation

   • Non-DR routers establish full adjacencies only with the DR and BDR.
   • Reduces the number of adjacencies from O(n²) to O(n).

2. Efficient LSA Flooding

   • The DR relays LSAs to all routers, preventing redundant transmissions.
   • Ensures all routers receive identical topology updates.

3. Failure Resilience

   • The BDR monitors the DR and takes over if it fails.
   • A new BDR is elected afterward.

Key Clarifications

• The DR does not make routing decisions—OSPF remains distributed.
• All routers maintain their own LSDB; the DR only optimizes synchronization.
• DR/BDR roles are per broadcast segment, not network-wide.

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Election Process: How DRs and BDRs Are Chosen

Election Criteria (Highest Wins)

1. OSPF Interface Priority

   • Default: 1 (configurable per interface).
   • Priority 0 = ineligible for DR/BDR roles.

2. Router ID (RID)

   • Used as a tie-breaker if priorities are equal.
   • Selected in this order:
     1. Manually configured RID.
     2. Highest IP on a loopback interface.
     3. Highest active interface IP.

Election Mechanics

• Hello Packets: Routers exchange Hellos to discover neighbors and participate in elections.
• Non-Preemptive: A new router with higher priority won’t replace an existing DR unless:
  • The DR fails.
  • The OSPF process resets.

Example: If Router A (priority 100) joins a network where Router B (priority 1) is already the DR, Router A won’t take over unless Router B fails.

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Failure Handling

Scenario	Outcome
DR fails	BDR promotes to DR; a new BDR is elected.
BDR fails	A new BDR is elected (DR remains unchanged).
Both DR and BDR fail	A new election occurs for both roles.
OSPF process resets	All adjacencies are rebuilt; new DR/BDR elections take place.

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Visualizing DR/BDR Impact

Without a DR (Full Mesh)

R1 ----- R2
 | \     / |
 |  \   /  |
 |   \ /   |
R3 ----- R4

• Problem: Each router forms 3 adjacencies (total: 6).
• Result: High bandwidth/CPU usage.

With DR/BDR (Hub-and-Spoke)

        (DR)
         R1
       / |  \
     R2  R3  R4
       \ |  /
       (BDR)

• Improvement: Non-DR routers form 2 adjacencies (total: 5).
• Result: Reduced overhead, faster convergence.

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Common Misconceptions

Misconception	Reality
The DR makes all routing decisions.	OSPF is distributed; the DR only optimizes LSA flooding.
Elections are global.	DR/BDR roles are per broadcast segment (e.g., per Ethernet VLAN).
Changing priority triggers re-election.	Elections are non-preemptive; changes only apply after a reset/failure.

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Practical Example: Enterprise LAN

Scenario: 10 routers on a single Ethernet switch.

Metric	Without DR	With DR/BDR
Adjacencies per router	9	2 (DR + BDR)
Total adjacencies	45	11
Bandwidth usage	High	Low
CPU load	High	Low

Outcome: The DR/BDR model reduces adjacencies by 75%, improving scalability.

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Key Takeaways

• Purpose: DRs/BDRs optimize OSPF on multi-access networks by reducing adjacencies and LSA flooding.
• Election Rules:
  • Highest interface priority wins.
  • Router ID breaks ties.
  • Priority 0 = ineligible.
• Behavior:
  • Elections are non-preemptive.
  • BDR promotes to DR if the DR fails.
• Scope: DR/BDR roles apply per broadcast segment, not the entire network.
• LSDB: All routers maintain an identical topology database; the DR only streamlines updates.

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Learn More

Official Resources

• RFC 2328: OSPF Version 2 (IETF)
• Cisco OSPF Design Guide

Further Reading

• OSPF vs. EIGRP: Comparison of Link-State and Distance-Vector Protocols
• Router ID Selection: Cisco Documentation
• Multi-Access Networks: IEEE 802.3 Ethernet Standards