Which communication protocol does IS-IS utilize?

IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol utilized within an Autonomous System in network routing. The communication protocol it primarily utilizes is CLNP (Connectionless Network Protocol).

CLNP is a network layer protocol from the OSI (Open Systems Interconnection) model that provides connectionless communication services. IS-IS was originally designed for routing in environments using CLNP, and it can carry various types of upper-layer protocol information, making it a versatile choice for routing in complex networks.

The other options do not fit as communication protocols that IS-IS employs. LACP (Link Aggregation Control Protocol) is used for bundling multiple network connections in parallel, STP (Spanning Tree Protocol) is a protocol for preventing loops in network topologies, and L2TP (Layer 2 Tunneling Protocol) is used for tunneling and does not pertain to the routing capabilities of IS-IS. Therefore, CLNP is the correct match for the communication protocol used by IS-IS.