As shown in the figure, four routers in the same LAN run IS-IS, among which R1 is DIS. Then R2, R3, and R4 establish adjacency relationships with R1 respectively, but no adjacency relationships are…

Question

Options

• ATRUE
• BFALSE

• A
  19% (10)
• B
  81% (42)

Explanation

In IS-IS on a broadcast (LAN) network, all routers form full adjacency relationships with every other router on the segment - not just with the DIS. So R2, R3, and R4 do establish adjacencies with each other, making the statement FALSE and option B correct.

Option A is wrong because it conflates IS-IS behavior with OSPF behavior. In OSPF, DROther routers only form full adjacencies with the DR and BDR, not with each other - but IS-IS has no such restriction; the DIS role is about generating pseudonode LSPs and flooding efficiency, not about limiting adjacency formation.

The key distinction: the IS-IS DIS reduces flooding overhead by acting as a pseudonode representative of the LAN, but every router on the broadcast segment still discovers and becomes adjacent to every other router via Hello packets.

Memory tip: Think of IS-IS on a LAN as a "full mesh of friends" - everyone knows everyone. OSPF's DR/BDR is the "hub-and-spoke" model where only the hub (DR/BDR) gets direct relationships. If the question mentions IS-IS + LAN, assume full mesh adjacency regardless of who the DIS is.

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