DR/BDR Concepts in OSPF – Full Explanation

✅ What is DR and BDR in OSPF?

DR (Designated Router): A router elected to manage OSPF routing updates in a multi-access network like Ethernet.

BDR (Backup Designated Router): A standby router that takes over the DR role in case of failure.

✅ Purpose of DR/BDR

To reduce OSPF overhead and unnecessary adjacencies in broadcast/multi-access environments.

Minimizes control traffic
Optimizes LSA flooding
Reduces CPU and memory usage

✅ Adjacency Efficiency Example

For 5 routers on a broadcast network:

Without DR: 10 adjacencies
With DR/BDR: Only 8 adjacencies (each router connects to DR and BDR only)

✅ Role of DR/BDR

Only DR and BDR form full adjacency with others. Other routers (called DROTHER) only form 2-way communication with each other.

✅ DR/BDR in Network Types

Network Type	DR/BDR Used?	Description
Broadcast	Yes	Ethernet LANs, Wi-Fi
NBMA	Yes	Frame Relay, ATM
Point-to-Point	No	Serial links
Point-to-Multipoint	No (default)	Each treated as separate P2P

✅ DR/BDR Election Process

Occurs per subnet when OSPF starts
Election criteria:
- Highest OSPF priority wins
- If tied, highest Router ID (RID)
- Priority 0 → not eligible

Election Example

Router	Priority	RID	Result
R1	1	1.1.1.1	BDR
R2	1	2.2.2.2	DR
R3	0	3.3.3.3	DROTHER

✅ Influencing DR/BDR Election

interface Gig0/0
 ip ospf priority 100

priority 0 = never DR/BDR
priority 1-255 = eligible

✅ Behavior on Failure

If DR fails → BDR becomes DR
New BDR elected from DROTHERs
BDR failure alone does not trigger re-election of DR

✅ LSA Handling by DR

DR collects LSAs from DROTHERs
Distributes LSAs to other routers
Minimizes flooding and improves efficiency

✅ OSPF States

Router	With	State
R1 (DROTHER)	R2 (DR)	Full
R1 (DROTHER)	R3 (BDR)	Full
R1	R4 (DROTHER)	2-Way only

✅ Show Commands

show ip ospf neighbor
show ip ospf interface Gig0/0

Sample Output:

Neighbor ID     Pri   State           DR/BDR        Interface
2.2.2.2         1     FULL/DR         2.2.2.2       Gig0/0
1.1.1.1         1     FULL/BDR        2.2.2.2       Gig0/0
3.3.3.3         0     2WAY/DROTHER    2.2.2.2       Gig0/0

✅ DR vs BDR vs DROTHER

Role	Full Adjacency With	Sends LSAs To	Receives LSAs From
DR	All routers	Everyone	Everyone
BDR	All routers	Standby	Everyone
DROTHER	Only DR & BDR	Only DR/BDR	DR/BDR

✅ Convergence and Real-World Use

DR/BDR increases convergence efficiency in multi-access networks
Not used in P2P links (faster, simpler)
Used in large LANs, Frame Relay hubs

✅ NBMA and DR/BDR

NBMA networks require manual neighbor config due to no multicast support:

router ospf 1
 neighbor 10.0.0.2 priority 100

✅ Summary Table

Concept	Value
DR	Designated Router – main router for LSAs
BDR	Backup for DR
DROTHER	Neither DR nor BDR
Used In	Broadcast, NBMA networks
Not Used In	Point-to-Point, Point-to-Multipoint (default)
Election Criteria	OSPF Priority → RID
Priority 0	Excludes router from election
DR Failure	BDR promoted → New BDR elected
Show Commands	`show ip ospf neighbor`, `show ip ospf interface`

OSPF DR/BDR Concepts Quiz

1. What is the role of a Designated Router (DR) in OSPF?

A. Acts as a backup if the main router fails B. Central router that reduces OSPF overhead by managing routing updates C. Handles routing only for point-to-point links D. Excludes routers from forming adjacencies

Correct answer is B. The DR manages routing info exchange to minimize adjacencies and optimize OSPF traffic.

2. How many adjacencies would be present in a network with 5 routers without DR/BDR?

A. 5 B. 8 C. 7 D. 10

Correct answer is D. Without DR/BDR, adjacencies = N*(N-1)/2 = 5*4/2 = 10.

3. Which routers form full adjacency with all others in OSPF multi-access networks?

A. DR and BDR B. DROTHER routers C. Only DR D. Only BDR

Correct answer is A. DR and BDR form full adjacencies with all routers; others only with DR and BDR.

4. In which network types is DR/BDR election NOT used by default?

A. Broadcast and NBMA B. Ethernet LANs C. Point-to-Point and Point-to-Multipoint D. Frame Relay networks

Correct answer is C. Point-to-point and point-to-multipoint links do not use DR/BDR by default.

5. What happens if the DR fails in an OSPF network?

A. A new DR election is triggered immediately among all routers B. The BDR becomes the new DR, and a new BDR is elected among DROTHER routers C. The network stops routing until the DR is manually replaced D. The DROTHER router with highest ID becomes DR

Correct answer is B. BDR takes over as DR, and a new BDR is elected without a full re-election.

6. Which parameter is used first to elect the DR or BDR?

A. Router ID B. IP address C. Interface status D. OSPF Priority

Correct answer is D. OSPF priority determines election first; Router ID breaks ties.

7. What priority value prevents a router from becoming DR or BDR?

A. 0 B. 1 C. 100 D. 255

Correct answer is A. Priority 0 excludes the router from DR/BDR election.

8. What adjacency state do DROTHER routers typically reach with DR and BDR?

A. 2-Way state B. Down state C. Full state D. Init state

Correct answer is C. DROTHER routers form full adjacencies only with DR and BDR.

9. Which command shows OSPF neighbors and their DR/BDR roles?

A. show ip route ospf B. show ip ospf neighbor C. show ip protocols D. show ip ospf database

Correct answer is B. This command lists OSPF neighbors with DR/BDR roles.

10. What is the state called when two routers are initially discovering neighbors in OSPF?

A. 2-Way B. Full C. Init D. Down

Correct answer is A. 2-Way is the initial state for neighbor discovery in OSPF.

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