Which communication protocol does IS-IS use?

A.
CLNP

B.
LACP

C.
STP

D.
L2TP

Explanation:
The Intermediate Systems to Intermediate System Routing Protocol (IS-IS) was originally designed to route the ISO Connectionless Network Protocol (CLNP) .
(ISO10589 or RFC 1142)
Adapted for routing IP in addition to CLNP (RFC1195) as Integrated or Dual IS-IS

A router is an Intermediate System (IS), and a host is an End System (ES). Accordingly, a protocol that provides communication between a host and a router is known as ES-IS, and a protocol that routers use to speak to each other (a routing protocol) is IS-IS (Figure 10-1). Whereas IP uses router discovery mechanisms, such as Proxy ARP, IRDP, or in the case of IPv6 NDP, or simply has a default gateway configured on hosts, CLNP uses ES-IS to form adjacencies between End Systems and Intermediate Systems.

Figure 10-1. Hosts are End Systems in ISO terminology, and routers are Intermediate Systems.