📘 CCNA 200-301 v1.1
3.4 Configure and verify single area OSPFv2
3.4.c Broadcast (DR/BDR selection)
1. What is a Broadcast Network in OSPF?
A broadcast network is a network type that allows a single packet sent by one device to be received by all devices on that network segment.
In OSPF, the most common example of a broadcast network is an Ethernet LAN.
- All routers on the same broadcast network can reach each other directly at Layer 2 (they are on the same subnet).
- OSPF treats broadcast networks as multi-access networks — meaning multiple routers can connect to the same network segment.
Examples of broadcast networks:
- Ethernet LANs (Fast Ethernet, Gigabit Ethernet, etc.)
2. Why DR and BDR Are Needed in Broadcast Networks
In OSPF, routers form adjacencies with other routers to exchange routing information.
If every router on a large LAN formed a full adjacency with every other router, it would cause too much OSPF traffic.
Let’s see why:
- Each OSPF router sends Hello, LSA, and LSU messages.
- If 10 routers formed full adjacencies with each other, that’s 10 × 9 = 90 adjacencies.
- That means lots of unnecessary LSA flooding and CPU overhead.
To avoid this, OSPF uses Designated Router (DR) and Backup Designated Router (BDR).
3. Role of DR and BDR
| Role | Description |
|---|---|
| DR (Designated Router) | Acts as the central point for OSPF updates on a broadcast network. All routers send LSAs to the DR instead of all routers. The DR then floods LSAs to all routers. |
| BDR (Backup Designated Router) | Takes over if the DR fails. It listens to all LSAs but doesn’t actively send them unless needed. |
| DROTHER routers | All other routers that are neither DR nor BDR. They form adjacencies only with DR and BDR, not with each other. |
So instead of many adjacencies, only a few are formed:
- All routers form adjacency with DR and BDR only.
This significantly reduces OSPF overhead on LAN segments.
4. How DR/BDR Election Works
When OSPF routers start up in a broadcast network, they elect a DR and BDR automatically.
The election happens based on the following rules:
🔸 Step 1: Priority Value
Each OSPF-enabled interface has a priority (range: 0–255).
- Default priority = 1
- Higher priority = more chance to become DR
- Priority 0 = the router cannot become DR or BDR
So routers with priority 0 will never be elected as DR or BDR.
🔸 Step 2: Router ID (RID)
If two or more routers have the same highest priority, then:
- The router with the highest Router ID (RID) becomes the DR.
If DR is already chosen and a new router with higher priority joins later, it does not preempt (does not replace the current DR).
The election only happens when:
- The network starts up, or
- The current DR/BDR fails or goes down.
5. Election Process (Simplified)
- All routers on the broadcast segment send Hello packets.
Each Hello packet includes:- Router ID
- Priority
- DR and BDR information
- Routers check Hello packets from neighbors.
- Based on the priority and Router ID:
- Router with highest priority becomes DR
- Router with second highest priority becomes BDR
- Other routers become DROTHERs.
6. DR/BDR Adjacency Relationships
| Relationship | Description |
|---|---|
| DR ↔ All | Full adjacency formed |
| BDR ↔ All | Full adjacency formed |
| DROTHER ↔ DR/BDR | Adjacency formed |
| DROTHER ↔ DROTHER | No adjacency formed |
So, DROTHER routers do not exchange LSAs directly with each other.
They communicate through the DR (and BDR as backup).
7. OSPF Neighbor States in Broadcast Network
In a broadcast network, OSPF routers go through these neighbor states during adjacency formation:
- Down → No Hellos received
- Init → Hello received but no two-way communication yet
- 2-Way → Bidirectional communication established
- At this point, DR and BDR election happens
- ExStart / Exchange / Loading / Full → Full adjacency with DR/BDR only
So DROTHER routers stay in 2-Way state with each other but go to Full with DR and BDR.
8. OSPF Hello Packet Fields for DR/BDR
Hello packets in broadcast networks include these important fields:
- Router Priority
- Designated Router (DR) IP address
- Backup Designated Router (BDR) IP address
These fields help routers recognize who is currently the DR and BDR on that network.
9. Configuration of DR/BDR
You can influence which router becomes DR or BDR by adjusting the OSPF interface priority.
Command:
R1(config)# interface GigabitEthernet0/0
R1(config-if)# ip ospf priority 200
- Priority range: 0–255
- Higher = more likely to become DR
0= never become DR/BDR
10. Verifying DR/BDR Status
Use the following commands:
🔹 Check OSPF neighbor relationships:
R1# show ip ospf neighbor
Output includes:
- Neighbor ID
- State (Full/DROTHER)
- Interface
- DR/BDR status
Example:
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 FULL/DR 00:00:33 192.168.1.2 Gig0/0
3.3.3.3 1 FULL/BDR 00:00:33 192.168.1.3 Gig0/0
🔹 Check interface OSPF details:
R1# show ip ospf interface GigabitEthernet0/0
Important fields:
- OSPF network type: Broadcast
- State: DR, BDR, or DROTHER
- Router Priority
- Designated Router & Backup Designated Router info
11. Key Points to Remember for CCNA Exam
| Concept | Key Information |
|---|---|
| DR/BDR election | Happens in broadcast and non-broadcast multi-access (NBMA) networks |
| Election criteria | Highest priority → Highest Router ID |
| Priority 0 | Excluded from DR/BDR election |
| Preemption | Does NOT occur after DR/BDR elected |
| DROTHER routers | Form adjacencies only with DR/BDR |
| Default priority | 1 |
| Interface command | ip ospf priority <value> |
| Verification | show ip ospf neighbor , show ip ospf interface |
12. Example Scenario (Simple IT Environment)
Let’s assume 3 routers (R1, R2, R3) connected to the same Ethernet switch.
| Router | OSPF Priority | Router ID | Result |
|---|---|---|---|
| R1 | 1 | 1.1.1.1 | DROTHER |
| R2 | 100 | 2.2.2.2 | DR |
| R3 | 50 | 3.3.3.3 | BDR |
R2 becomes the Designated Router (DR) because it has the highest priority.
R3 becomes Backup Designated Router (BDR).
R1 is DROTHER.
✅ Summary
| Term | Meaning | Importance |
|---|---|---|
| Broadcast network | Network where all routers can reach each other directly (like Ethernet) | Common in LANs |
| DR (Designated Router) | Central OSPF router for LSAs on broadcast networks | Reduces LSA flooding |
| BDR (Backup DR) | Backup for DR | Takes over if DR fails |
| DROTHER | All other routers | Form adjacency only with DR/BDR |
| Priority | Determines DR/BDR election | Higher = more likely |
| No preemption | DR/BDR not replaced unless failure | Important for stability |
