The previous article in this series covered the scalability challenges multiaccess OSPF segments face — unnecessary adjacencies and duplicated LSA flooding. The Designated Router (DR) is OSPF’s solution to both problems. This article covers exactly what the Designated Router does, how it and the Backup Designated Router (BDR) are elected, and walks through a complete, internally consistent failure-and-recovery scenario so the election rules are clear in practice, not just in the abstract.
What the Designated Router Does
On a multiaccess network, a DR and BDR are elected as soon as OSPF initializes on the segment. The DR becomes the central collection and distribution point for Link-State Advertisements (LSAs): every other router on the segment sends its LSAs to the DR, and the DR reflooads them out to everyone else. This means a topology change only needs to be flooded once through the DR, rather than every router flooding directly to every other router — the mechanism that solves the adjacency-scaling problem covered in the previous article.
The BDR listens passively to this entire exchange and independently maintains full knowledge of the segment, without actively performing DR duties itself. If the DR stops sending Hello packets — the sign OSPF uses to detect a router failure — the BDR promotes itself to DR immediately, without needing to run a fresh election for that role.
Every other router on the segment that is neither DR nor BDR is called a DROTHER. DROTHERs form full adjacencies only with the DR and BDR, exchanging LSAs with them via the multicast address 224.0.0.6 (AllDRouters). Between two DROTHERs, the relationship never goes past the 2-Way state — they acknowledge each other’s Hello packets but don’t exchange full topology information directly, since there’s no need to when the DR already has the complete picture.
DR and BDR Election Criteria
OSPF uses two criteria, in strict order, to elect the DR and BDR on a multiaccess segment:
1. Interface Priority
Every OSPF-enabled interface has a priority value from 0 to 255, with a default of 1 on multiaccess broadcast interfaces. The router with the highest priority becomes DR; the router with the second-highest priority becomes BDR.
Priority 0 is a special case: a router with priority 0 can never become DR or BDR — it will always remain a DROTHER, regardless of its Router ID. Serial interfaces default to priority 0, which is one reason DR/BDR election doesn’t happen on point-to-point serial links in the first place — there’s no multiaccess segment there to elect a DR for.
Because the default priority is 1 on every multiaccess interface, most networks don’t have any priority differentiation out of the box, which means the election almost always falls through to the second criterion.
2. Router ID (Tiebreaker)
When priorities are tied — which, by default, they usually are — the router with the highest Router ID becomes DR, and the router with the second-highest Router ID becomes BDR. The Router ID is determined in this order:
- A manually configured Router ID, set with the
router-idcommand under the OSPF process. - If not manually configured, the highest IP address among the router’s loopback interfaces.
- If no loopback interfaces exist, the highest IP address among the router’s active physical interfaces.
On a network running OSPFv3 for IPv6 where the router has no IPv4 addresses configured at all, the Router ID must be set manually, since OSPFv3 has no IPv4 address to fall back on and will not start without one.
The election process itself begins the moment the first OSPF-enabled interface comes up on the segment — as soon as the network command activates OSPF on that interface, or the router powers on with OSPF already configured — and it typically completes within a few seconds.
DR Election Is Non-Preemptive
This is one of the most commonly tested points on the CCNA exam: once a DR and BDR are elected, the election does not run again just because a new, higher-priority or higher-RID router joins the segment. The existing DR keeps its role until it actually fails — its OSPF process stops, the interface goes down, or the router itself goes offline. A newly arriving router, no matter how high its priority or Router ID, simply joins as a DROTHER and waits.
Worked Scenario: DR/BDR Behavior Through Multiple Failures
To make the election rules concrete, here’s a complete, step-by-step scenario across four routers — R1, R2, R3, and R4 — all sharing one Ethernet segment with default priority (1) on every interface, so Router ID decides everything. Assume R1 has the highest Router ID, followed by R2, then R3, then R4.
Initial election: R1, having the highest Router ID, becomes DR. R2, with the second-highest Router ID, becomes BDR. R3 and R4 are DROTHERs.
R1 (the DR) fails. Per the non-preemptive promotion rule, R2 — the existing BDR — is promoted directly to DR with no new vote required for that specific role. A new BDR election then runs among the remaining DROTHERs, R3 and R4. Since R3 has the higher Router ID of the two, R3 becomes the new BDR. At this point: R2 = DR, R3 = BDR, R4 = DROTHER.
R2 (now the DR) also fails. The current BDR, R3, is promoted to DR. A new BDR election runs among whatever DROTHERs remain — at this point, only R4 is left, so R4 becomes BDR by default. At this point: R3 = DR, R4 = BDR, no DROTHERs remain on the segment.
R1 rejoins the network. Even though R1 originally had the highest Router ID in the entire segment, the non-preemptive election rule means it does not reclaim DR or BDR. R1 simply joins as a DROTHER. The final state: R3 = DR, R4 = BDR, R1 = DROTHER.
This sequence illustrates the two rules that matter most in practice: a BDR failure promotion never triggers a full re-election of the DR role itself, and a returning or newly joined router — regardless of priority or Router ID — never displaces an already-elected DR or BDR.
Verifying DR/BDR Roles
The show ip ospf interface command on any router reveals its own OSPF role on a given segment, along with the identities of the current DR and BDR:
Router# show ip ospf interface GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 192.168.1.3/24, Area 0
Process ID 1, Router ID 3.3.3.3, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DROTHER, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 192.168.1.1
Backup Designated router (ID) 2.2.2.2, Interface address 192.168.1.2The State line shows this router’s own role (DR, BDR, or DROTHER), while the Designated Router and Backup Designated Router lines identify which routers currently hold those roles by Router ID and interface address.
Verifying DR/BDR Adjacencies
The show ip ospf neighbor command shows the adjacency state with each neighbor on the segment. On a multiaccess network, expect to see one of these states:
- FULL/DR — a full adjacency with the segment’s DR; both routers actively exchange Hello packets, updates, queries, replies, and acknowledgements.
- FULL/BDR — a full adjacency with the segment’s BDR, functioning the same way as FULL/DR.
- FULL/DROTHER — seen from the perspective of the DR or BDR, describing its full adjacency with a non-DR/BDR router.
- 2-WAY/DROTHER — the relationship between two DROTHERs. They’ve exchanged Hello packets and recognize each other as neighbors, but haven’t synchronized their link-state databases with each other — because there’s no need to, since the DR already holds the complete picture.
FULL and 2-Way are both perfectly normal states depending on which two routers are involved. Any other state persisting for more than a few seconds — INIT, EXSTART, or EXCHANGE, for example — usually signals an adjacency formation problem worth investigating with debug ip ospf adj.
Controlling DR/BDR Placement with Priority
Because the default election almost always falls to Router ID — which is essentially arbitrary from a design perspective, since it’s usually just whichever loopback or interface address happens to be highest — relying on the default behavior can leave a low-capacity or poorly-placed router serving as DR on a busy segment. The standard mitigation is to explicitly configure priorities so the intended DR and BDR are chosen deliberately rather than by coincidence:
! Intended DR — highest priority on the segment
Router-A(config)# interface GigabitEthernet0/0
Router-A(config-if)# ip ospf priority 200
! Intended BDR — second-highest priority
Router-B(config)# interface GigabitEthernet0/0
Router-B(config-if)# ip ospf priority 100
! Routers that should never become DR or BDR
Router-C(config)# interface GigabitEthernet0/0
Router-C(config-if)# ip ospf priority 0Because the election is non-preemptive, changing priority values on routers that are already part of an active OSPF network doesn’t trigger a new election by itself. To force a fresh election after adjusting priorities, the OSPF process needs to be restarted:
Router# clear ip ospf processThis is disruptive — it tears down every OSPF adjacency on that router while the process restarts — so it’s generally reserved for planned maintenance windows rather than applied casually in a live production network.
Frequently Asked Questions
What function does the OSPF Designated Router perform?
The DR’s core function is disseminating LSAs to every other router on a multiaccess network segment. It acts as the central collection and distribution point: routers send their LSAs to the DR (via multicast to 224.0.0.6), and the DR floods them out to the whole segment (via multicast to 224.0.0.5). This eliminates the need for every router to flood directly to every other router.
Beyond flooding, the DR also generates the Type 2 Network LSA representing the multiaccess segment itself and the routers attached to it, and maintains a synchronized link-state database across the segment. Note that the DR is a distinct role from an Area Border Router (ABR), which summarizes routes between areas, or an Autonomous System Boundary Router (ASBR), which redistributes external routes — the DR does neither of those things.
Does the OSPF DR election happen on point-to-point links?
No. DR and BDR election only occurs on broadcast and non-broadcast multi-access (NBMA) network types, where more than two routers could potentially share the segment. A point-to-point link, by definition, connects exactly two routers, so there’s no adjacency-scaling problem for a DR to solve, and no election takes place.
If the DR fails, does a full new election happen?
No — this is a common point of confusion. When the DR fails, the existing BDR is promoted directly to DR without a fresh vote for that role. Only the now-vacant BDR position triggers a new election, held among the remaining DROTHERs.
Can a router with a higher Router ID take over as DR from an already-elected router?
No. OSPF’s DR election is non-preemptive. Once a DR and BDR are in place, they keep those roles regardless of any new router that joins with a higher priority or Router ID. The only way a new DR is elected is if the current DR actually fails.
How do I verify which router is the DR on a segment?
Run show ip ospf interface on any router connected to the segment. The output shows the current DR and BDR by Router ID and interface address, along with this router’s own state (DR, BDR, or DROTHER).
Conclusion
The Designated Router exists to solve the two multiaccess OSPF problems covered in the previous article: unnecessary adjacency counts and duplicated LSA flooding. Election comes down to interface priority first, then Router ID as a tiebreaker — with priority 0 permanently excluding a router from the role. The behavior that trips up the most students is what happens during failure: a BDR promotion to DR is automatic and immediate, a fresh election only fills the now-open BDR slot, and the non-preemptive rule means no returning or newly joined router — however high its priority or Router ID — ever displaces an already-elected DR or BDR.
