IS-IS Network Design Solutions

Defining the System ID

By now, it is clear that the SysID is one of three key components of an NSAP address, which is required to enable IS-IS routing on a router. The other components are the Area ID and the NSEL. According to ISO 10589, the SysID can be of variable length between 1 to 8 bytes. However, Cisco's implementation of IS-IS uses a fixed 6-byte length in conformance with the GOSIP 2.0 standard. It is probably not by coincidence that the 6-byte length specified by GOSIP 2.0 matches the length of a LAN MAC address. Therefore, you can use one of the LAN MAC addresses on a router as its SysID, essentially embedding a MAC address (a Layer 2 address) in the NSAP, which is a Layer 3 address. Of course, on an IP router with many LAN interfaces, ...