In ISO terminology, packets are referred to as Protocol Data Units (PDUs). There are 3 categories of IS-IS packets:

• IS-IS Hello Packets (IIHs): These packets establish and maintain adjacencies between IS-IS neighbors.
• Link State PDUs (LSPs): These packets are responsible to distribute routing information between IS-IS nodes.
• Sequence Number PDUs (SNPs): These packets control the distribution of LSPs. SNPs provide mechanism to synchronize LSDBs between routers in the same area.

Each packet category consists of various types of packets; each type is assigned a Type Number. All IS-IS packets are multicast on LAN. For Level-1 ISs, the packets are sent to 01-80-C2-00-00-14, and for Level-2 ISs, the packets are sent to 01-80-C2-00-00-15. 

The following table shows IS-IS packet types:

Category	Packet Type	Type Number
Hello	LAN Level-1 Hello	15
	LAN Level-2 Hello	16
	Point-to-point Hello	17
LSP	Level-1 LSP	18
	Level-2 LSP	20
SNP	Level-1 Complete SNP	24
	Level-2 Complete SNP	25
	Level-1 Partial SNP	26
	Level-2 Partial SNP	27

Generic IS-IS Packet Header 

Each IS-IS PDU mentioned in above table has an 8-byte header. The following packet capture shows the basic format:

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