Blog Posts in July 2025
IS-IS 3-Way Handshake and the Power of SHOULD
Yesterday, I mentioned that a Cisco router running pre-standard IS-IS 3-way handshake (this is why you need it) interoperates with multiple implementations of RFC 5303. How’s that possible, and does it matter whether you configure the ancient Cisco routers (release 15.x) to use IETF 3-way handshake instead of the “proprietary” one?
TL&DR: It SHOULD NOT matter, but the more I explore the RFCs, the more I’m amazed anything works at all.
I took a trip to the Wireshark land to figure out the details (you can download the capture file):