OSPF DR/BDR – Designated Router Election, LSA Flooding & Adjacency Optimisation

1. The Problem DR/BDR Solves

In OSPF, routers that share a network segment must form adjacencies — synchronised Link State Database (LSDB) relationships — before they can exchange routing information. On a broadcast multi-access network (like an Ethernet LAN), every router can hear every other router. Without any special mechanism, every router would form a full adjacency with every other router on the same segment. 

  Without DR/BDR — 5 routers on one Ethernet segment:

  R1 ━━━━━━ R2         All 5 routers form full adjacencies
   ┃ ╲     ╱ ┃         with every other router
   ┃  ╲   ╱  ┃
   ┃   ╲ ╱   ┃         Adjacency count = N×(N-1)/2
   ┃   ╱ ╲   ┃                         = 5×4/2 = 10 adjacencies
   ┃  ╱   ╲  ┃
   ┃ ╱     ╲ ┃         Each adjacency requires:
  R3 ━━━━━━ R4         • Hello/DBD/LSR/LSU/LSAck exchange
       ╲   ╱           • Separate LSDB synchronisation
        ╲ ╱            • Ongoing keepalive traffic
        R5              → 10 full adjacencies = massive overhead

  With DR/BDR — same 5 routers:

         R2 (DR)        R3, R4, R5 are DROTHER
        ↗ ↑ ↖           They only form Full adjacency with DR and BDR
       /  |  \          They form 2-Way (not Full) with each other
      /   |   \
    R1  R4   R5         Adjacency count = 2×(N-2) + 1 = 7 full adjacencies
      \   |   /         (each DROTHER → DR and → BDR)
       \  |  /          Plus DR ↔ BDR = 1
        ↘ ↓ ↙           Total Full adjacencies: 7 ← significant reduction
        R1 (BDR)

Related pages: OSPF Overview | OSPF Neighbor States | OSPF Single-Area Config | OSPF Areas & LSAs | OSPF Single-Area Lab | OSPF Multi-Area Lab

2. Adjacency Math — With and Without DR/BDR

The reduction in full adjacencies is the core reason DR/BDR exists. The formula for full mesh adjacencies (no DR) is N×(N-1)/2.

Routers (N) Without DR/BDR
N×(N-1)/2		 With DR/BDR
2(N-2)+1		 Adjacencies Saved
3 3 3 0 (no benefit for 3 routers)
5 10 7 3
10 45 17 28
20 190 37 153
50 1225 97 1128
Exam tip: For CCNA exams the most tested adjacency count is for 5 routers: without DR = 10 adjacencies; with DR/BDR = 8 adjacencies (the original page states 8 — the formula 2(N-2)+1 gives 7 Full adjacencies, but counting all pairwise relationships including 2-Way gives 8 total relationships. In practice exam questions accept either 7 or 8; the key point is the dramatic reduction compared to 10).

3. DR, BDR, and DROTHER — Roles Defined

Role What It Is Full Adjacency With Sends LSUs To Receives LSUs From
DR
(Designated Router)		 The elected "hub" router on the segment. Central point for all OSPF LSA exchange. Represents the segment in the LSDB as a Network LSA (Type 2). All routers on segment (DR + BDR + all DROTHERs) All routers via 224.0.0.5 (AllSPFRouters multicast) DROTHERs (sent to 224.0.0.6 AllDRRouters)
BDR
(Backup Designated Router)		 Standby DR — listens to all exchanges, maintains full LSDB, ready to take over instantly if DR fails. Does not actively distribute LSAs while DR is alive. All routers on segment (same as DR) Takes over DR's role on DR failure All LSUs sent by DROTHERs to 224.0.0.6
DROTHER Any router that is neither DR nor BDR. The majority of routers on a segment are DROTHERs. Only DR and BDR (Full); with other DROTHERs only 2-Way DR and BDR only via 224.0.0.6 (AllDRRouters multicast) DR via 224.0.0.5 (DR redistributes to all)

OSPF Multicast Addresses

Multicast Address Name Who Listens Who Sends
224.0.0.5 AllSPFRouters All OSPF routers on the segment DR (when distributing LSAs to all routers) and Hello packets
224.0.0.6 AllDRRouters Only DR and BDR DROTHERs (when sending LSUs to DR/BDR)
The critical insight: DROTHERs never send LSAs directly to other DROTHERs. They send to 224.0.0.6 (only DR and BDR receive). The DR then re-floods the LSA to 224.0.0.5 (all routers receive). This hub-and-spoke LSA distribution is what eliminates redundant flooding.

4. OSPF Network Types — Where DR/BDR Applies

Network Type DR/BDR Used? Typical Interface Hello / Dead Timers Notes
Broadcast Yes Ethernet (GigabitEthernet, FastEthernet) 10s / 40s Default on Ethernet; DR/BDR elected automatically; supports multicast
NBMA
(Non-Broadcast Multi-Access)		 Yes Frame Relay, ATM (legacy) 30s / 120s No multicast support — neighbors must be manually configured; DR election still occurs
Point-to-Point No Serial (HDLC, PPP), GRE tunnels 10s / 40s Only two routers; DR/BDR unnecessary; always goes directly to Full state
Point-to-Multipoint No (default) Frame Relay hub-and-spoke (manually set) 30s / 120s Treats each PVC as a separate P2P link; no DR/BDR; avoids NBMA complexity
Loopback No Loopback interface N/A Always advertised as host route /32
Changing the OSPF network type: You can override the default with ip ospf network <type> on the interface. For example, configuring ip ospf network point-to-point on an Ethernet interface disables DR/BDR election entirely — useful when only two routers are connected on that segment.

5. DR/BDR Election Algorithm — Step by Step

The election occurs per segment when OSPF first starts. Once a DR and BDR are elected, they remain in place unless they fail — this is the non-preemption rule (covered in Section 6). 

  Election algorithm (runs during OSPF initialisation):

  Step 1: Wait for the dead interval (to hear from all routers on segment)
          Default: 40 seconds on Ethernet

  Step 2: Compare OSPF interface priority values
          Range: 0–255   Default: 1
          Priority 0 = router is INELIGIBLE (never becomes DR/BDR)
          HIGHEST priority wins

  Step 3: If priority is tied → compare Router IDs
          Highest Router ID wins

  Step 4: Router with highest wins = DR
          Router with second highest = BDR
          All remaining = DROTHER

  Router ID selection (highest wins):
  1st: Manually configured  → router ospf 1 / router-id 4.4.4.4
  2nd: Highest loopback IP  → interface Loopback0 / ip address 3.3.3.3 255.255.255.255
  3rd: Highest active interface IP
  See: OSPF Overview — Router ID Selection

Election Example — 4 Routers on a Segment

Router OSPF Priority Router ID Election Result Reasoning
R1 200 1.1.1.1 DR Highest priority (200) → wins DR regardless of RID
R2 100 2.2.2.2 BDR Second-highest priority (100) → BDR
R3 1 4.4.4.4 DROTHER Lower priority than R1/R2; highest RID among DROTHERs but priority takes precedence
R4 0 3.3.3.3 DROTHER (ineligible) Priority 0 = permanently excluded from DR/BDR election

Tiebreaker Example — Equal Priorities

Router OSPF Priority Router ID Election Result
R1 1 (default) 1.1.1.1 DROTHER
R2 1 (default) 2.2.2.2 BDR (second-highest RID)
R3 1 (default) 3.3.3.3 DR (highest RID)

6. Non-Preemption — The Critical Behaviour to Understand

OSPF DR/BDR elections are non-preemptive. Once a DR and BDR are elected, they keep their roles even if a new router with a higher priority or RID joins the segment later. This is one of the most important and most tested DR/BDR concepts. 

  Non-preemption scenario:

  Initial state (R1=DR, R2=BDR, R3=DROTHER — all priority 1):
  R1 (RID 1.1.1.1, priority 1) → DR
  R2 (RID 2.2.2.2, priority 1) → BDR
  R3 (RID 3.3.3.3, priority 1) → DROTHER

  R4 joins with priority 255, RID 4.4.4.4:
  Expected? You might think R4 should become DR.
  Actual behaviour: R4 becomes DROTHER.
  R1 stays DR. R2 stays BDR.
  R4's superior priority is IRRELEVANT as long as R1 and R2 are alive.

  Why? Stability — constant DR re-elections would disrupt the network.
  Every re-election triggers LSDB re-synchronisation and convergence events.

  To force R4 to become DR:
  Method 1: Reload R1 and R2 (they must leave and re-join the segment)
  Method 2: Use "clear ip ospf process" on all routers to force new election
  Method 3: Plan ahead — set R4's priority BEFORE connecting it to the segment
Practical implication: If you want a specific router to be DR (e.g., your most powerful router), configure its priority before starting OSPF or before connecting it to the segment. After the election, changing priority has no immediate effect — you must clear the OSPF process on all routers in the segment to force a new election.

7. DR/BDR Failure and Recovery Behaviour

  Normal operation:
  R1(DR) ←→ R2(BDR) ←→ R3(DROTHER) ←→ R4(DROTHER)
  All DROTHERs send LSUs to DR and BDR via 224.0.0.6
  DR distributes to all via 224.0.0.5

  Scenario A — DR (R1) fails:
  ┌─────────────────────────────────────────────────────┐
  │ 1. R2 (BDR) detects DR failure via Dead timer expiry │
  │ 2. R2 IMMEDIATELY becomes the new DR                │
  │    (no new election needed — BDR was already ready) │
  │ 3. New BDR election runs among DROTHER routers only │
  │    R3 or R4 wins BDR based on priority/RID          │
  │ 4. Network continues routing without interruption   │
  └─────────────────────────────────────────────────────┘
  Result: R2=DR, R3 or R4=BDR, remaining=DROTHER
  Non-preemption still applies — if R1 recovers, it becomes DROTHER

  Scenario B — BDR (R2) fails:
  ┌───────────────────────────────────────────────────────┐
  │ 1. DR (R1) continues operating normally               │
  │ 2. No DR re-election triggered                        │
  │ 3. New BDR election runs among DROTHER routers only   │
  │ 4. Winning DROTHER becomes new BDR                    │
  └───────────────────────────────────────────────────────┘
  Result: R1=DR (unchanged), R3 or R4=new BDR

  Scenario C — DROTHER fails:
  No election impact. DR and BDR continue operating.
  Network topology changes trigger LSU exchange as normal.

8. LSA Flooding Optimisation by DR

The DR's most important function is to act as the central distribution point for Link State Advertisements (LSAs) on the segment. Without DR, every router would flood every LSA to every neighbour, causing exponential retransmission. 

  LSA flooding WITHOUT DR (5 routers, each floods independently):

  R3 detects link change → sends LSU to R1, R2, R4, R5
  R1 receives, floods back to R2, R4, R5
  R2 receives from R3 AND R1, floods to R4, R5 again...
  Each router floods independently → duplicate LSAs → inefficiency

  LSA flooding WITH DR:

  Step 1: R3 detects link change
  Step 2: R3 sends LSU to 224.0.0.6 (AllDRRouters — only DR/BDR receive)
  Step 3: DR receives, sends LSAck back to R3
  Step 4: DR floods LSU to 224.0.0.5 (AllSPFRouters — all routers receive)
  Step 5: All routers (including BDR and DROTHERs) update LSDB
  Step 6: All routers send LSAck to 224.0.0.6 (DR/BDR acknowledge receipt)

  Result: Each LSA is flooded exactly ONCE from DR — no duplicates
  BDR also receives and stores all LSAs, ready to take over as DR

Type 2 Network LSA — DR's Advertisement

The DR generates a special Type 2 (Network) LSA for the broadcast segment it manages. This LSA lists all routers attached to the segment and allows the rest of the OSPF domain to understand the segment's topology without each individual router advertising it separately.

  • Only the DR generates the Type 2 Network LSA for the segment
  • If the DR changes, the new DR generates a new Type 2 LSA
  • The DR uses its own Router ID as the Link State ID in the Type 2 LSA
  • See OSPF Areas and LSAs for the complete LSA type reference

9. OSPF Adjacency States — 2-Way vs Full

Understanding which routers reach which state with which neighbours is essential for reading show ip ospf neighbor output correctly. 

  OSPF neighbour state progression (simplified):

  Down → Init → 2-Way → Exstart → Exchange → Loading → Full

  For DROTHER ↔ DR or DROTHER ↔ BDR:
  Goes all the way to → FULL (complete LSDB synchronisation)

  For DROTHER ↔ DROTHER:
  Stops at → 2-WAY (bidirectional hellos confirmed; NO LSDB exchange)

  For any Point-to-Point link:
  Always goes to → FULL (no DR/BDR involved)
Neighbour Pair State Reached LSDB Exchanged? Why
DROTHER ↔ DR Full Yes Full adjacency required for LSA exchange through DR
DROTHER ↔ BDR Full Yes BDR must have complete LSDB to take over on DR failure
DR ↔ BDR Full Yes DR and BDR must be synchronised for seamless failover
DROTHER ↔ DROTHER 2-Way No DROTHERs route traffic through DR; no direct LSDB sync needed

See OSPF Neighbor States for a detailed walkthrough of all OSPF states (Down through Full).

10. Configuration — Influencing DR/BDR Election

! ── Set OSPF interface priority ──────────────────────────────────────────
! Higher priority = more likely to become DR (range 0-255, default 1)
! Priority 0 = never eligible for DR or BDR

Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip ospf priority 200    ! Highest → will become DR after election

Router(config)# interface GigabitEthernet0/1
Router(config-if)# ip ospf priority 100    ! Second → will become BDR after election

Router(config)# interface GigabitEthernet0/2
Router(config-if)# ip ospf priority 0      ! Zero → DROTHER only; never DR or BDR

! ── Set Router ID (for tiebreaking) ──────────────────────────────────────
! Highest RID wins when priority is tied
! Configure BEFORE starting OSPF for deterministic RID

Router(config)# router ospf 1
Router(config-router)# router-id 10.0.0.1   ! Explicitly set RID

! ── Force re-election after priority change ───────────────────────────────
! Priority changes do NOT take effect immediately (non-preemption rule)
! Must clear OSPF process on ALL routers in the segment to force new election

Router# clear ip ospf process
! Warning: This resets ALL OSPF adjacencies on the router
! Yes/No prompt: yes

! ── Change OSPF network type (disables DR/BDR on Ethernet) ───────────────
Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip ospf network point-to-point    ! Disables DR/BDR on this interface
! Useful when only 2 routers share an Ethernet segment

! ── NBMA manual neighbor configuration ────────────────────────────────────
! On NBMA networks (Frame Relay), multicast is not available
! Must manually specify neighbors and their priorities
Router(config)# router ospf 1
Router(config-router)# neighbor 10.0.0.2 priority 100   ! Specify neighbour IP and priority

11. Verification Commands — Full Output Reference

! ── View all OSPF neighbors with DR/BDR roles ────────────────────────────
Router# show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
4.4.4.4         255   FULL/DR         00:00:35    10.1.1.4        GigabitEthernet0/0
2.2.2.2         100   FULL/BDR        00:00:38    10.1.1.2        GigabitEthernet0/0
3.3.3.3         0     2WAY/DROTHER    00:00:32    10.1.1.3        GigabitEthernet0/0

! Reading this output:
! Neighbor ID  = Router ID of the neighbour
! Pri          = OSPF priority of the neighbour
! State        = Adjacency state / Role (FULL/DR, FULL/BDR, 2WAY/DROTHER)
! Dead Time    = How long until dead timer expires (counts down from 40s)
! Address      = Neighbour's IP on the shared segment
! Interface    = Our local interface

! ── View DR/BDR info for a specific interface ─────────────────────────────
Router# show ip ospf interface GigabitEthernet0/0

GigabitEthernet0/0 is up, line protocol is up
  Internet Address 10.1.1.1/24, Area 0, Attached via Network Statement
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DROTHER, Priority 1
  Designated Router (ID) 4.4.4.4, Interface address 10.1.1.4   ← DR info
  Backup Designated Router (ID) 2.2.2.2, Interface address 10.1.1.2  ← BDR info
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
  oob-resync timeout 40
  Hello due in 00:00:05
  Supports Link-local Signaling (LLS)
  Neighbor Count is 3, Adjacent neighbor count is 2
    Adjacent with neighbor 4.4.4.4 (Designated Router)    ← Full with DR
    Adjacent with neighbor 2.2.2.2 (Backup Designated Router)  ← Full with BDR

! Key fields to note:
! State DROTHER  → this router is neither DR nor BDR
! Designated Router: shows DR's RID and IP address
! Adjacent neighbor count: 2 (only Full with DR and BDR, not with 3.3.3.3)
! Neighbor Count: 3 (sees 3 neighbours; 2-Way with the DROTHER)

! ── Verify Router ID ──────────────────────────────────────────────────────
Router# show ip ospf
 Routing Process "ospf 1" with ID 1.1.1.1
 ...

! ── Check OSPF routing information ───────────────────────────────────────
Router# show ip route ospf
Router# show ip ospf database      ! View full LSDB including Type 2 Network LSAs

12. NBMA Networks and DR/BDR

Non-Broadcast Multi-Access (NBMA) networks (historically Frame Relay and ATM) present a special challenge: multiple routers share the same logical network but the physical medium does not support multicast or broadcast. This breaks OSPF's automatic neighbour discovery (which relies on multicast Hello packets to 224.0.0.5). 

  NBMA hub-and-spoke (Frame Relay):

              Hub (R1) — typically becomes DR
             ╱    ╲
        Spoke1    Spoke2
        (R2)      (R3)

  Problem: R2 and R3 can reach R1 (hub) via PVCs
           But R2 and R3 CANNOT directly reach each other
           (no PVC between them — NBMA asymmetry)

  DR election on NBMA creates a problem:
  If R2 or R3 becomes DR, the hub (R1) cannot reach the DR directly
  Solution: Force the hub (R1) to be DR by giving it highest priority
! NBMA configuration — Hub router (R1) becomes DR
Router-Hub(config)# router ospf 1
Router-Hub(config-router)# neighbor 10.0.0.2 priority 0   ! Spoke1 excluded from DR election
Router-Hub(config-router)# neighbor 10.0.0.3 priority 0   ! Spoke2 excluded from DR election

! On hub interface — set high priority to ensure hub is DR
Router-Hub(config)# interface Serial0/0
Router-Hub(config-if)# ip ospf priority 100    ! Hub becomes DR

! On spoke interfaces — set priority 0 to prevent spokes from becoming DR
Router-Spoke1(config)# interface Serial0/0
Router-Spoke1(config-if)# ip ospf priority 0

! On spoke routers — manually specify the hub as OSPF neighbour
Router-Spoke1(config)# router ospf 1
Router-Spoke1(config-router)# neighbor 10.0.0.1   ! Hub's NBMA IP

13. Common Troubleshooting Scenarios

Symptom Likely Cause Diagnostic & Fix
Router stuck in 2-Way state with all neighbours on broadcast network No DR/BDR has been elected — all routers may have priority 0, or election failed show ip ospf interface Gi0/0 — verify State; check all routers don't have priority 0; clear ip ospf process to force re-election
Wrong router is DR despite setting high priority Non-preemption — priority change after election has no effect; the current DR stays until it fails Use clear ip ospf process on all routers in the segment simultaneously to force a new election with the new priorities
All routers show DROTHER in show ip ospf neighbor Possible mismatch in OSPF area, hello/dead timers, authentication, or subnet mask — neighbors are not forming properly show ip ospf interface on all routers — verify Area ID, timers, and authentication match; check OSPF Neighbour Adjacency Troubleshooting
OSPF adjacency flapping on NBMA network Spoke cannot reach DR directly; asymmetric PVC topology; hub not configured as DR Set hub priority to highest; set spoke priority to 0; manually configure neighbors with neighbor command
Slow convergence after DR failure BDR promotion is fast but new BDR election among DROTHERs takes a full wait timer (40 seconds) Tune OSPF timers: reduce dead/hello intervals; consider BFD (Bidirectional Forwarding Detection) for faster failure detection

14. Key Points & Exam Tips

  • DR/BDR exists only on broadcast and NBMA network types. Point-to-point and point-to-multipoint links have no DR/BDR.
  • Without DR/BDR: adjacencies = N×(N-1)/2. With DR/BDR: Full adjacencies = 2(N-2)+1. For 5 routers: 10 without, ~7-8 with.
  • Election order: (1) Highest OSPF interface priority (0–255, default 1). (2) Highest Router ID (tiebreaker). Priority 0 = never eligible.
  • Non-preemption: Once elected, DR/BDR keep their roles even if a higher-priority router joins. A new election only occurs when DR or BDR fails. Use clear ip ospf process to force re-election.
  • DROTHER adjacency states: DROTHER ↔ DR = Full. DROTHER ↔ BDR = Full. DROTHER ↔ DROTHER = 2-Way (not Full).
  • Multicast: DROTHERs send LSUs to 224.0.0.6 (AllDRRouters — only DR/BDR receive). DR floods to 224.0.0.5 (AllSPFRouters — all receive). Hellos use 224.0.0.5.
  • DR failure: BDR immediately becomes DR (no election). New BDR elected from DROTHERs. BDR failure alone: DR stays. New BDR elected from DROTHERs.
  • DR generates the Type 2 Network LSA representing the broadcast segment in the LSDB.
  • Set priority with ip ospf priority <0-255> on the interface. Verify with show ip ospf interface <int> and show ip ospf neighbor.
  • NBMA: Must manually configure neighbors because no multicast; force hub to be DR with highest priority and spokes to priority 0.

Related pages: OSPF Overview | OSPF Neighbor States | OSPF Single-Area Config | OSPF Areas & LSAs | show ip route | OSPF Single-Area Lab | OSPF Multi-Area Lab | Troubleshooting OSPF Adjacency

15. OSPF DR/BDR Concepts Quiz

1. A network has 6 routers on a single Ethernet segment running OSPF with all default priorities. After the election, an engineer adds R7 with ip ospf priority 255 to the same segment. R7 has the highest priority of any router on the segment. What role does R7 take?

Correct answer is C. OSPF's non-preemption rule is one of the most important and most tested DR/BDR concepts. Once a DR and BDR are elected on a segment, they retain their roles indefinitely as long as they remain operational — regardless of any new router joining with higher priority or RID. The election only runs again when the DR fails (BDR promotes to DR and a new BDR is elected) or when the BDR fails (new BDR elected from DROTHERs). This is by design: re-elections are expensive (trigger LSDB re-synchronisation and convergence events) and should only happen on real failures. To force R7 to become DR, you would need to set its priority before connecting it, or run clear ip ospf process on all routers in the segment to force a new election.

2. Four routers (R1, R2, R3, R4) are on the same Ethernet segment. R1 has priority 1 and RID 1.1.1.1, R2 has priority 1 and RID 2.2.2.2, R3 has priority 0 and RID 9.9.9.9, R4 has priority 1 and RID 4.4.4.4. Which routers become DR and BDR?

Correct answer is B. The election algorithm first filters out ineligible routers (priority 0 = never DR/BDR). R3 has priority 0 so it is immediately excluded regardless of its RID (9.9.9.9). Among the eligible routers (R1, R2, R4 — all priority 1), the tiebreaker is the highest Router ID. R4 has RID 4.4.4.4 (highest) → DR. R2 has RID 2.2.2.2 (second highest) → BDR. R1 has RID 1.1.1.1 (lowest) → DROTHER. R3 with its impressive RID of 9.9.9.9 is irrelevant — priority 0 is an absolute exclusion, not modified by the RID. This is a common exam trap: high RID doesn't help if priority is 0.

3. When a DROTHER router needs to send a routing update (LSU) to the rest of the OSPF network, to which multicast address does it send the update, and why not to 224.0.0.5?

Correct answer is D. The OSPF multicast model on broadcast networks is a hub-and-spoke pattern: DROTHERs send LSUs to 224.0.0.6 which only DR and BDR receive. The DR then floods that LSU to 224.0.0.5 which all OSPF routers receive. If DROTHERs sent directly to 224.0.0.5, every DROTHER would also receive every other DROTHER's LSU and be required to process it — creating the same N²/2 flooding problem that DR/BDR was designed to solve. The two-address model is the key to understanding how DR/BDR actually reduces overhead. Hellos are sent to 224.0.0.5 by all routers (including DROTHERs) because neighbor discovery must reach all routers. Only LSUs and LSAcks use the 224.0.0.6 / 224.0.0.5 separation.

4. An engineer wants to verify that R1 is acting as the DR on its GigabitEthernet0/0 interface. Which command provides the most direct confirmation, and what field in the output confirms the DR role?

Correct answer is A. show ip ospf interface GigabitEthernet0/0 gives the most complete per-interface OSPF status. Key fields: "State DR" confirms this router is the DR. "Designated Router (ID) <RID>, Interface address <IP>" shows who the DR is (if this is the DR, it shows its own RID and IP). "Backup Designated Router (ID) <RID>, Interface address <IP>" shows the BDR. "Adjacent neighbor count" shows how many Full adjacencies exist (if DR, this equals all other routers). show ip ospf neighbor (option B) shows neighbour states from this router's perspective — it shows other routers' roles (FULL/DR, FULL/BDR) but the DR flag there refers to the neighbour's role, not this router. To confirm this router's own role, use show ip ospf interface. Use show ip route ospf to verify OSPF routes and show ip ospf database to examine the full LSDB including Type 2 Network LSAs.

5. An OSPF network has R1 as DR, R2 as BDR, and R3/R4 as DROTHERs. R1 fails. What is the sequence of events, and what is the final OSPF topology?

Correct answer is C. DR failure triggers a two-step recovery, not a full re-election. Step 1: The BDR (R2) has been monitoring all LSA exchanges and maintaining a full LSDB. When R1's dead timer expires, R2 immediately transitions from BDR to DR — this is an instant promotion, not an election. No waiting period, no voting. Step 2: Now there is no BDR. A new BDR election runs among the remaining DROTHERs (R3 and R4 only). The winner is determined by the same priority/RID algorithm. This BDR election does involve a wait period (Wait timer = Dead interval, 40 seconds by default) to ensure all routers on the segment can participate. During this brief wait, R2 (new DR) continues to operate normally. The reason for this two-phase design: the BDR's immediate promotion means there is no service interruption for the DR role — routing continues without interruption. Only finding a new BDR takes time.

6. An engineer uses show ip ospf neighbor and sees two neighbours in "2WAY/DROTHER" state and two in "FULL/DR" and "FULL/BDR" state. What does the "2WAY/DROTHER" state specifically mean about the relationship between this router and the DROTHER neighbours?

Correct answer is B. The 2-Way state between DROTHERs is completely normal and expected — it is NOT a problem or misconfiguration. It means: (1) Both routers have sent Hello packets that the other received and acknowledged (bidirectional communication confirmed). (2) Each router sees itself in the other's Hello packet neighbour list. (3) No LSDB exchange has occurred or will occur between these two DROTHERs — OSPF only forms Full adjacencies (and performs LSDB synchronisation) between DROTHERs and the DR/BDR. The DROTHER/DROTHER relationship stays at 2-Way by design — all LSDB synchronisation happens through the DR as the hub. This is the entire point of having a DR: DROTHERs synchronise with the DR/BDR, and the DR ensures everyone has consistent LSDB, without needing DROTHER-to-DROTHER full adjacencies. The state "2WAY/DROTHER" means: "we are in 2-Way state AND the neighbour's role is DROTHER."

7. A network engineer needs to prevent a particular router from ever becoming DR or BDR on an Ethernet interface, while ensuring it still participates in OSPF routing. What is the correct configuration?

Correct answer is D. ip ospf priority 0 on an interface is the precise mechanism to exclude a router from DR/BDR election on that specific interface while keeping it as a fully-participating OSPF router. The router will still: send and receive Hello packets, form Full adjacencies with the DR and BDR, exchange LSDB with DR/BDR, install OSPF routes, and forward traffic. It simply will never be elected as DR or BDR on that interface — permanently, regardless of the priorities and RIDs of other routers on the segment. Common use case: access-layer routers in a campus network that should not carry DR responsibilities (leave that to the distribution-layer switches). Option B (point-to-point network type) disables DR/BDR entirely for the link but also changes OSPF behaviour in other ways and is typically only used when exactly two routers share an Ethernet segment.

8. An engineer is deploying OSPF over a legacy Frame Relay NBMA hub-and-spoke network. There is one hub router (R1) and three spoke routers (R2, R3, R4). The spokes cannot reach each other directly — only through the hub. What OSPF design decision is critical, and why?

Correct answer is A. NBMA OSPF design has a critical constraint that makes it unique: on an NBMA network, not all routers can reach each other directly — only through the hub. In the hub-and-spoke topology, if R2 (a spoke) were elected DR, R3 and R4 (other spokes) would need to send LSUs to the DR (R2) but they have no PVC to R2 — only to R1 (hub). This means R3 and R4 could never form full adjacencies with the DR, completely breaking OSPF operation. The solution is mandatory: configure the hub (R1) as DR by giving it the highest OSPF priority on the NBMA interface, and configure all spokes with priority 0 so they cannot become DR or BDR. Additionally, since NBMA doesn't support multicast, all routers must statically configure their neighbours with neighbor <IP> commands. Option B (point-to-multipoint) is actually a valid alternative — it treats each PVC as a separate point-to-point link and avoids DR/BDR entirely — but the question asks about NBMA mode specifically.

9. An engineer checks show ip ospf interface Gi0/0 and sees "Adjacent neighbor count is 2" but "Neighbor Count is 4". This router is a DROTHER. What does this output reveal about the OSPF topology on this segment?

Correct answer is C. The distinction between "Neighbor Count" and "Adjacent neighbor count" in show ip ospf interface output is a direct reflection of DR/BDR operation. Neighbor Count: all routers from which valid Hello packets have been received (bidirectional Hello = 2-Way or higher). This is 4 — the router sees 4 other OSPF routers on the segment. Adjacent neighbor count: the subset of neighbours that have reached Full state (complete LSDB synchronisation). For a DROTHER, this is exactly 2 — only the DR and BDR. The other 2 neighbours are also DROTHERs; with them, this router only maintains 2-Way state. This is 100% correct and expected OSPF operation. If this number were different (e.g., 0 or 1), then there would be a problem. The output also lists which specific neighbours are "Adjacent with" — they will be labelled "(Designated Router)" and "(Backup Designated Router)".

10. An engineer configures ip ospf priority 200 on R1's Ethernet interface but show ip ospf neighbor still shows R3 as DR and R2 as BDR, despite both having priority 1. What must the engineer do to make R1 the DR, and what is the risk of this action?

Correct answer is B. This scenario demonstrates a critical operational reality of OSPF non-preemption. Changing ip ospf priority 200 on R1 takes effect in R1's Hello packets immediately — other routers will see R1's new priority of 200 in subsequent Hellos. However, because of non-preemption, R3 (current DR) and R2 (current BDR) will not yield their roles just because a higher- priority router joined. The election is over and the result stands. To force a new election that honours R1's new priority, the OSPF process must be cleared on all routers on the segment: clear ip ospf process. This resets all adjacencies to Down state and restarts the entire OSPF initialisation sequence including a fresh DR/BDR election. The risk: all OSPF routes are withdrawn temporarily; traffic may be lost while adjacencies reform and LSDBs re-synchronise (typically takes 30–60 seconds). Best practice: plan DR/BDR before deployment — set priorities before OSPF starts to avoid needing forced re-elections in production.

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