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IS-IS Overview Configuring IS-IS
page 3-10 OmniSwitch 6800/6850/9000 Advanced Routing Configuration Guide December 2007
IS-IS Packet Types
IS-IS transmits data in little chunks known as packets. There are four packet types in IS-IS. They are:
• Intermediate System-to-Intermediate System Hello (IIH)—Used by routers to detect neighbors and
form adjacencies.
• Link State Packet (LSP)—Contains all the information about adjacencies, connected IP prefixes, OSI
end system, area address, etc. There are four types of LSPs: Level-1 pseudo node, Level-1 non-pseudo
node, Level-2 pseudo node, and Level-2 non-pseudo node.
• Complete Sequence Number PDU (CSNP)—Contains a list of all the LSPs from the current data-
base. CSNPs are used to inform other routers about LSPs that may be outdated or missing from their
own database. This ensures that all routers have the same information and are synchronized.
• Partial Sequence Number PDU (PSNP)—Used to request an LSP(s) and acknowledge receipt of an
LSP(s).
IS-IS Areas
IS-IS allows collections of contiguous networks and hosts to be grouped together as an area. Each area
runs a separate copy of the basic link state routing algorithm (usually called SPF). This means that each
area has its own topological database as explained in the previous section.
IS-IS Areas
Area 01
Area 03
Area 04
Area 02
L1
L1
L1
L1
L1/L2
L1/L2
L1/L2
L1/L2