IETF RFC 5303 PDF

The IS-IS routing protocol (Intermediate System to Intermediate System, ISO 10589) requires reliable protocols at the link layer for point-to-point links. As a result, it does not use a three-way handshake when establishing adjacencies on point-to-point media. This paper defines a backward-compatible extension to the protocol that provides for a three-way handshake. It is fully interoperable with systems that do not support the extension.

Additionally, the extension allows the robust operation of more than 256 point-to-point links on a single router.

This extension has been implemented by multiple router vendors; this paper is provided to the Internet community in order to allow interoperable implementations to be built by other vendors.