ISIS is an OSI protocol. It is accessed directly at data link layer. Router ID cannot be defined in isis. ISIS use system ID.
ISIS routers need to be assigned OSI addresses, which they use as a Router ID to create network structure.
For eg: for an Network Entity Title (NET) 49.001a.1122.3344.5566.00, The area is 49.001a. The router's system ID is 1122.3344.5566. The easiest way to figure this out is to start from the right and work towards the left. The last two numbers of the NET are the NSEL; they are always 00 on a router. The next 12 numbers (separated into 3 groups of 4 numbers) are the system ID. On Cisco routers, the system ID is always this length—6 bytes. Anything to the left of the system ID is the area ID.
so how to set router-id in Service providers NW routers , as you know if we do not need to monitor the p routers in MPLS VPN network( IGP :ISIS ) we will not run BGP on the P routers so how to configure ( Hard Code )router-id on them ?