IS-IS introduces the 3-way handshake mechanism on P2P links to ensure a reliable data link layer.
Based on ISO 10589, the IS-IS 2-way handshake mechanism uses Hello packets to set up P2P adjacencies between neighboring devices. When a device receives a Hello packet from the other end, it regards the other end as Up and sets up an adjacency with it. However, this mechanism has some serious shortcomings.
When two or more links exist between two devices, an adjacency can still be set up where one link is Down and the other is Up in the same direction. The parameters of the other link are used in SPF calculation. As a result, a device that does not detect any fault along the faulty link will continue trying to forward packets over the link.
The 3-way handshake mechanism resolves these problems on P2P links. In 3-way handshake mode, a device regards a neighbor Up and sets up an adjacency with it only after confirming that the neighbor has received the packet that the device sends.
In addition, the 3-way handshake mechanism uses the 32-bit Extended Local Circuit ID field, which extends the original 8-bit Extended Local Circuit ID field and the limit of only 255 P2P links.
By default, the IS-IS 3-way handshake mechanism is implemented on P2P links.