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IS-IS Network Design Solutions by Scott Sturgess, Abe Martey

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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, ...

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