Hello @jaideepchoudhary ,
>> Is system ID of 0000.0000.0000 is invalid in ISIS or not recommended ?
As far as I know it is invalid.
The System ID is actually part of a CNLS address of ISO OSI protocol stack.
CLNS addresses were thought to be a substitute for IPv4.
But:
in creating the standards many different address formats are allowed as a result of this CLNS addresses have variable sizes ranging from 6 bytes to 20 bytes.
ISDN / PSTN E164 can be used.
ATM had a 20 bytes format to create SVCs
As a result of this.
Usually System ID can be built by converting the IPv4 address of the loopback as System ID must be unique in area for L1 nodes and unique in whole domain for L1/l2 or L2 IS-IS nodes.
Several management systems of SDH equipment uses CLNS natively and IS-IS started as a link state to route CLNS packets. LAter it was extended to be able to route also IPv4 prefixes and then IPv6 prefixes.
In very old IOS we needed to enable clns routing to use IS-IS, this is not needed anymore. But be aware that IS-IS is not an IP based protocol but it can route IPv4, IPv6 address families and there is also support for multi topology.
Hope to help
Giuseppe
