IS-IS in OSI protocol stack

From CT3

Numerous sources on the Internet claim that IS-IS runs on top of OSI’s Connectionless Network Protocol (CLNP). This is not the case; although IS-IS and CLNP share the same layer-2 Service Access Point (SAP), OSI provides an additional field (Network Layer Protocol Identifier; NLPID) in the first byte of the layer-3 header. The identification of a layer-3 OSI protocol is thus performed based on layer-2 information (DSAP = 0xFE) and the first byte of the layer-3 header, which has the following values:

NLPID value	OSI protocol
0x81	CLNP (Connectionless Network Protocol)
0x82	ES-IS: End system (host) to Intermediate System (router) protocol
0x83	IS-IS: Intermediate System to Intermediate System protocol

IS-IS is therefore a separate network-layer protocol and does not rely on CLNP for datagram transport (IP routing protocols encapsulate their packets into IP or UDP datagrams).

The relationship between various OSI protocols and their comparison with the IP protocol stack, where the layer-3 protocol demultiplexing relies exclusively on the values in the layer-2 header is shown in the following diagram:

Various protocols in OSI and IP protocol stacks

Additional Resources

Configuring Configuring Cisco Routers for Integrated IS-IS (CISIS) course:

Building Scalable Cisco Internetworks (BSCI) course:

Did you know?

NIL developed the first commercially available advanced technical training covering Cisco IOS implementation of IS-IS.
This course was for several years the only course available to Cisco's internal audiences and its Service Provider customers in Europe.
The IS-IS course developed by NIL later became part of Cisco's Service Provider training curriculum.
NIL's experts have helped several large Service Providers design and deploy IS-IS networks.