Why this exists
On a multi-access segment, if every router formed a full adjacency
with every other router, you’d get n(n-1)/2 adjacencies and massive
duplicate flooding. The DR solves this: routers become fully
adjacent only with the DR and BDR, cutting the adjacency count and
centralizing LSA distribution.
The scaling problem in numbers
Why n(n-1)/2 matters
On a segment with 5 routers, full mesh would mean 5×4/2 = 10
adjacencies, each flooding the same LSAs. With a DR, every router
is adjacent only to the DR and BDR → adjacencies grow roughly
linearly (n) instead of exponentially. The DR becomes the single
point that collects and redistributes link-state info for the segment.
Where it applies
DR/BDR only on multi-access
- Broadcast (Ethernet) → DR/BDR elected.
- NBMA (Frame Relay, X.25, ATM) → DR/BDR elected.
- Point-to-point / point-to-multipoint → no election (links
are effectively one-to-one, so a DR adds nothing).
Election order
Priority first — Router ID only breaks ties
- Highest interface priority wins (range 0–255, default 1).
- If priority is tied → highest Router ID wins.
The slide that says “the DR is the router with the highest Router ID”
is incomplete: priority is always checked first. RID is only the
tie-breaker.
- Priority 0 → the router can never become DR or BDR (permanent
DROTHER). - Highest priority = DR, second highest = BDR.
- Everything else = DROTHER.
Non-preemptive behaviour
The election does NOT re-run automatically
Once a DR is elected, a new router joining later with a higher
priority will not take over. The DR only changes if the current DR
fails (then the BDR is promoted and a new BDR is elected). To force a
change you must reset the OSPF process. This is deliberate — it avoids
needless re-convergence every time a router joins.
The role of the BDR
Why a backup exists
The BDR listens to everything the DR does but stays silent unless
the DR fails. If the DR goes down, the BDR is already synchronized and
is promoted instantly — no waiting for a full re-election to rebuild
state. A fresh BDR is then chosen among the DROTHERs.
Multicast roles
Who talks to whom
- DROTHERs → DR/BDR using 224.0.0.6 (AllDRouters).
- DR → all routers using 224.0.0.5 (AllSPFRouters).
DROTHERs send their updates to the DR/BDR on .6; the DR relays the
consolidated link-state information back out to everyone on .5.
Connection to the LSDB
The DR generates the Type 2 LSA
On a multi-access segment, the DR originates the Type 2
(Network) LSA that represents the segment and lists the routers
attached to it. No DR means no Type 2 LSA — which is why point-to-
point links don’t produce one. (Detail in
06 - LSA Types & the LSDB.)
Concept anchor
Tip
Interface priority is what you tune to control the outcome, and the
election takes place as routers reach the Two-Way state. Setting
priority to 0 is the clean way to keep a router out of the DR role
on a given segment. (ip ospf priority <0-255>is the interface
command behind this.)