Which of the following descriptions of the IS-IS neighbor relationship establishment process are correct? (Multiple choice)

Question

Options

• AFor Level-1 routers, the area numbers may be inconsistent.
• BThe two-way handshake mechanism can be used to establish neighbor relationships on the P2P link, which is faster and more stable than the three-way handshake mechanism.
• CThe IS-IS neighbor relationship is established through the three-way handshake mechanism on the P2P link, the neighbor IDs of both parties are Up, which means that the neighbor relationship is successfully established between the two parties.
• DOn broadcast networks, DIS needs to be elected.

• A
  19% (9)
• B
  9% (4)
• C
  72% (34)

Explanation

C is correct because IS-IS uses a three-way handshake on P2P links - both routers must explicitly list each other's System ID in their Hello PDUs before the adjacency reaches the Up state, confirming bidirectional reachability. D is correct because broadcast networks require a DIS (Designated Intermediate System) election to reduce flooding overhead and generate pseudonode LSPs, analogous to OSPF's DR/BDR election.

A is wrong because Level-1 routers must share the same area ID to form an adjacency - area number consistency is a hard requirement for L1 neighbors, not optional. B is wrong because IS-IS does not use a two-way handshake on P2P links; it uses three-way, and the three-way mechanism is preferred precisely because it is more reliable (it confirms both sides see each other), not less stable.

Memory tip: Map IS-IS to OSPF parallels - "P2P uses 3-way handshake" (like OSPF's full adjacency confirmation) and "broadcast elects a DIS" (like OSPF's DR). For Level-1, remember: same area, same level - any mismatch blocks the adjacency.

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