Which Routers a DRother Exchanges LSAs With
Short answer
AC A DRother forms FULL adjacencies with the BDR and DR on a broadcast network, exchanging link state and route information only with them.
Question
Key Takeaways
This tests OSPF DR/BDR adjacency rules; the trap is thinking a DRother exchanges full LSAs with all neighbours on the segment, when DRother-to-DRother contact is limited to implicit Hello-based discovery.
In OSPF broadcast or NBMA networks, a DRother router forms FULL adjacencies and exchanges link state plus route information only with the DR and BDR (options A and C), never with other DRothers.
Common mistake: Choosing B, 'All OSPF neighbors' — DRothers on the same broadcast segment do form adjacencies with the DR and BDR only, and merely exchange Hellos with other DRothers, never full link state or routing information.
Expert Analysis
Reading the constraint The stem asks which routers a DRother exchanges link state information and route information with, and it is explicitly multi-select ("Choose all that apply"). That wording narrows the scope to OSPF broadcast/NBMA segments, where the DR/BDR roles exist at all; on point-to-point links there is no DR, no BDR and no DRother, so the roles named in the options only make sense on a multi-access network.
Eliminating option B "All OSPF neighbors" is the trap. On a broadcast segment a DRother forms FULL adjacencies only with the DR and the BDR; it deliberately does not form a full adjacency with other DRothers. Their mutual exchange is reduced to implicit relationship discovery through Hello packets — they never exchange full LSAs or routing information with each other. So B overstates the neighbour set and must be discarded.
Eliminating option D Option D names DRother itself. A DRother certainly exchanges information with other DRothers only in the limited implicit sense described above, not as full link-state/route exchange, and the stem asks about the routers whose role partners a DRother is — so D is not a correct selection.
Confirming A and C The DRother builds a FULL adjacency with both the DR (C) and the BDR (A), sending its LSUs to the AllDRouters multicast group and receiving the DR's synchronised LSDB and routing updates. Both neighbours are therefore valid answers: the DR distributes and acknowledges LSAs, and the BDR holds an identical adjacency so it can take over immediately if the DR fails. Hence the correct set is A and C — exactly two selections, with no fixed count imposed by the stem.
Exam Strategy
On multi-select OSPF adjacency questions, first identify whether the stem restricts the scenario to a broadcast/NBMA segment, since DR/BDR roles only apply there. Then apply the rule that a DRother forms FULL adjacency with the DR and BDR only, and downgrade any option claiming exchange with every neighbour to an implicit Hello-only relationship. Eliminate self-referential options like 'DRother' unless the question explicitly asks about DRother-to-DRother contact.
Frequently Asked Questions
Why does a DRother not exchange full LSAs with other DRothers?
What happens to DRother adjacency if the DR fails?
Does this DR/BDR rule apply on point-to-point links?
Community Votes
50% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 3 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 3
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e9b92a4 👍 1✅ Correct answers: A and C Explanation In an OSPF broadcast or NBMA network, routers are elected into specific roles: DR (Designated Router) BDR (Backup Designated Router) DRother (all other routers) The way link-state and routing information is exchanged depends on these roles. How a DRother router exchanges information A DRother router: Forms full OSPF adjacencies (FULL state) only with the DR and the BDR Sends its LSUs to the DR, which then floods them to other routers Does not exchange LSAs directly with other DRother routers Therefore, a DRother exchanges link-state and route information with: ✅ DR ✅ BDR
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Fmoura17 👍 1The correct answers are A and C. A. BDR ✅ DROther routers establish Full adjacency with the BDR and exchange complete link state and routing information with it. The BDR serves as a backup in case the DR fails. C. DR ✅ DROther routers establish Full adjacency with the DR and exchange complete link state and routing information with it. The DR is responsible for collecting and distributing LSAs to all routers on the segment.
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ButcherCapybara 👍 1a DRother router exchanges link-state information and route information primarily with the Designated Router (DR) and the Backup Designated Router (BDR).
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