3 - IS-IS - Intermediate System to Intermediate System / ISO 10589
IS-IS (Common Part) PCI
Intradomain-Routing-Protocol Discriminator (1 byte)
Length Indicator (1 byte) (ISO 10589)
Version / PID Extension (1 byte)
ID Length (1 byte)
res.(1) (3 bits)
Type (5 bits)
Version (1 byte)
res.(2) (2 bytes)
Variable Part (n bytes)