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Routing · Cisco / Network Engineer What's an OSPF DR/BDR and why are they elected?
On multi-access networks (Ethernet broadcast), OSPF elects a Designated Router (DR) and Backup DR (BDR) to reduce LSA flooding. Without DR, every router would establish full adjacencies with every other router on the segment (n*(n-1)/2 adjacencies). With DR, all routers form adjacencies with DR only — n adjacencies. Election: highest OSPF priority wins (default 1). Tie-broken by highest router ID. Priority 0 = ineligible. DR/BDR not used on point-to-point links.
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Related Routing questions
Routing
Q. What's the difference between OSPF, EIGRP, and BGP?
OSPF (link-state, IGP, open standard, RFC 2328) — runs Dijkstra SPF inside each area, fast convergence, hierarchical with areas. EIGRP (advanced distance-vector, IGP, originally Cisco-proprietary now open) — uses DUAL al…
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Q. Explain OSPF area types — backbone, regular, stub, totally stubby, NSSA.
Backbone (area 0) — must be present, all other areas connect to it. Regular area — accepts all LSA types (1, 2, 3, 4, 5). Stub area — blocks Type 5 (external LSAs); ABR injects default route. Totally stubby (Cisco propri…
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Q. EIGRP DUAL algorithm — explain Successor and Feasible Successor.
Successor — the route with the lowest metric to a destination, installed in the routing table. Feasible Successor (FS) — a backup route that satisfies the Feasibility Condition: FS reported distance < Successor feasible …
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