Hello @Ratheesh mv ,
pre RFC 5303 behaviuor is described in RFC Abstract:
>>
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.
So pre RFC5303 implementations have the following limitations:
no three way handshake is actually performed on point to point links by IS-IS itself
Note: Cisco may have had a 3 way handshake using standard circuit IDs.
The second important limitation is the original circuit descriptors allowed to describe up to 256 different p2p links ( a single octet was used to describe the circuit ID) on a single router and this limit is too low for modern routers that can have thousands of logical interfaces and hundreds of physical interfaces.
Your wireshark packet captures confirm the use of extended circuit IDs descriptors when using ietf option.
>> When I did packet capture with isis three-way-handshake ietf there were extended local circuit ID , Neighbor system-ID and neighbor extended local circuit ID fields in the TLV 240 but these fields were not present with isis three-way-handshake cisco.
Hope to help
Giuseppe
