OSPF network types and DR election
Point-to-point and broadcast network types, DR/BDR election behavior and non-preemption, and their timer and next-hop differences.
Network types
OSPF network types on the exam
| Property | Broadcast (Ethernet default) | Point-to-point |
|---|---|---|
| DR/BDR election | Yes — one DR + one BDR per segment | No — no election |
| Hello / dead | 10 s / 40 s | 10 s / 40 s |
| Multicast use | 224.0.0.5 all routers, 224.0.0.6 DR/BDR | All traffic to 224.0.0.5 |
| Next-hop behavior | Next hop = originating router (not the DR) | Direct neighbor |
| Use | Multiaccess segments | Serial, GRE tunnels, PtP Ethernet |
Force point-to-point on a dedicated link and influence DR election on the LAN — Privileged EXEC → Interface configuration
Cisco IOS / IOS XE
configure terminal interface gigabitethernet0/1 ip ospf network point-to-point exit interface gigabitethernet0/2 ip ospf priority 100 end
show ip ospf interface gi0/1 show ip ospf neighbor
- Interface shows NETWORK POINT_TO_POINT — no DR listed.
- On broadcast interfaces, the neighbor with highest priority holds DR.
- Priority 0 marks a router that will never become DR — set it on all but your intended DR/BDR pair.
Changes device configuration.
DR/BDR election
On a broadcast segment, every router would otherwise maintain adjacency with every other — n(n−1)/2 adjacencies. The DR (designated router) collects and re-floods LSAs so each router fully synchronizes with the DR and BDR only. Election: highest ip ospf priority (default 1; 0 = ineligible), tie broken by highest RID.
DR election is non-preemptive: once a DR exists, a new router with higher priority does NOT take over — the current DR keeps its role until it fails or the segment resets. This surprises everyone once.
1.1.1.1 0 2-WAY/DROTHER 00:00:39 10.0.0.4 Gi0/2DROTHER-to-DROTHER stays 2-WAY; only DR/BDR reach FULL with this observer.2.2.2.2 100 FULL/DR 00:00:35 10.0.0.2 Gi0/2Highest priority — the DR.3.3.3.3 50 FULL/BDR 00:00:37 10.0.0.3 Gi0/2Next in line — the BDR.- Segment
- R1 pri 100 (was DR, now rebooting), R2 pri 50, R3 pri 1
- While R1 is down, R2 (50) becomes DR and R3 becomes BDR.
- R1 returns — election is non-preemptive → R2 keeps DR.
- R1 takes a DROTHER role until the current DR fails again.
The DR is 'first with highest priority at election time', not 'highest priority on the segment'.
Timers and verification
OSPF default timers
| Parameter | Broadcast / point-to-point | Non-broadcast (NBMA) |
|---|---|---|
| Hello interval | 10 s | 30 s |
| Dead interval | 40 s | 120 s |
| Retransmit LSA | 5 s | 5 s |
| LSA refresh | 30 min (originator) | 30 min |
Hello/dead must match between neighbors; mismatched timers alone prevent adjacency.
Common mistakes
- Expecting preemption — a higher-priority newcomer does NOT replace an established DR.
- Thinking point-to-point links elect a DR — they don't; no DR fields appear.
- Assuming the DR becomes the next hop — routes still point at the originating router.
- Forgetting hello/dead consistency — timers must match for the adjacency regardless of type.
Observation: Neighbors on an Ethernet segment both stay 2-WAY; routes exchange only partially.
- One side set to point-to-point, the other broadcast — type mismatch
- Both have priority 0 — no eligible DR
- Timers mismatched (but then they wouldn't reach 2-WAY)
| Check | Indicates |
|---|---|
show ip ospf interface on both ends | Reveals network-type mismatch — the classic cause of 2-WAY limbo. |
| Priorities on the segment | All zeros means no DR can be elected. |
show ip ospf neighbor states | 2-WAY between DROTHERs is normal; stuck 2-WAY where FULL was expected suggests a network-type problem, but FULL alone does not prove types match on every segment. |
Correction: Make network types match (broadcast/broadcast or point-to-point/point-to-point) and give at least one router nonzero priority.
Neighbors reach FULL/DR and FULL/BDR roles respectively.
Related resources
Sources
- Cisco CCNA 200-301 v1.1 Exam Topics (official PDF) — reviewed 2026-11-02
- RFC 2328 — OSPF Version 2 — reviewed 2026-11-02
- Cisco — IP Routing: OSPF Command Reference — reviewed 2026-11-02
Targets CCNA 200-301 v1.1 objectives. Supplemental blocks are labeled beyond exam scope.