Which of the following statements is correct?

Question

Options

• ABoth R2 and R3 will generate Level-1 LSPs with ATT set.
• BR1 does not have the LSP generated by R4, so R1 can only communicate with R4 through the default route.
• CBoth R2 and R3 will generate LSPs of Level 1-2 with ATT set.
• DR2 and R3 learn default routes from each other, and a routing loop occurs in the network.

• A
  77% (27)
• B
  3% (1)
• C
  14% (5)
• D
  6% (2)

Explanation

In IS-IS, a Level 1-2 (L1/L2) router sets the ATT (Attached) bit specifically in its Level-1 LSPs to signal to Level-1-only routers that it has connectivity to the Level-2 backbone. Since R2 and R3 are both L1/L2 routers, they each independently advertise this attachment by setting ATT=1 in their L1 LSPs - making option A correct.

C is wrong because the ATT bit is only carried in Level-1 LSPs, not in Level-2 LSPs. L1/L2 routers generate separate L1 and L2 LSPs; the ATT bit has no meaning in L2 LSPs (L2 routers already have full backbone visibility).

D is wrong because L1/L2 routers do not install default routes based on the ATT bit - only Level-1-only routers do. R2 and R3, participating fully in L2, have complete routing information and do not create a default-route loop with each other.

B is wrong because the network topology in this scenario places R4 within the same L1 area as R1, so R1 does have R4's L1 LSP and can reach R4 directly via intra-area routing - not only through a default route.

Memory tip: Think "ATT bit = Level-1 LSP only" - the bit is an announcement to L1-only routers, so it lives in the L1 LSP. If the answer mentions ATT in L2 or "L1-2 LSPs," it's a trap.

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