On an IS-IS network, two directly connected routers establish a neighbor relationship through the three-way handshake mechanism by default…

The mechanism for establishing an IS-IS neighbor relationship is dependent on the network type. While a three-way handshake is indeed used on broadcast networks to ensure mutual reachability before forming a full adjacency, Point-to-Point (P2P) networks use a simpler two-way handshake mechanism by default. On a P2P link, an adjacency is formed as soon as two routers have successfully exchanged P2P Hello packets, which is a two-step process. Therefore, the statement that a three-way handshake is used "regardless of the network type" is incorrect.

The "True" option is incorrect because it inaccurately generalizes the adjacency formation process. The IS-IS protocol optimizes neighbor discovery based on the link type. The three-way handshake is specifically required for the multi-access nature of broadcast networks (e.g., Ethernet) to reliably establish bidirectional communication and elect a Designated Intermediate System (DIS). This complexity is unnecessary on P2P links, where a two-way exchange is sufficient.

1. Huawei HCIP-Datacom-Advanced Routing & Switching Technology V1.0 Training Material.

Reference Section: Chapter "IS-IS Principles"

Sub-section "IS-IS Neighbor Relationship Establishment".

Content: The official training material explicitly distinguishes between the two processes. It states

"On a P2P link

two routers can establish a neighbor relationship as long as they can receive Hello packets from each other. This process is called a two-way handshake mechanism." It then contrasts this with broadcast networks: "On a broadcast network

IS-IS neighbor establishment requires a three-way handshake mechanism... This ensures that the two neighbors have established a bidirectional relationship." This directly contradicts the question's claim.

2. RFC 1195 - Use of OSI IS-IS for routing in TCP/IP and dual environments.

Reference Section: Section 3.4

"Adjacency Establishment and Maintenance".

Content: This foundational RFC describes the different IS-IS Hello (IIH) PDUs for different media. The Point-to-Point IIH PDU structure and the associated state machine imply a two-way recognition for adjacency formation. In contrast

the LAN IIH PDU and the rules for processing it on a broadcast circuit (described in ISO 10589

which RFC 1195 builds upon) necessitate the three-way exchange to confirm that a router has seen itself in its neighbor's Hello packet before declaring the adjacency "Up". The existence of two distinct Hello PDU types and processes for P2P and LAN environments confirms that a single mechanism is not used for both.