Four general packet types (PDUs) are communicated through IS-IS, appearing at both Level 1 and Level 2. They are described below.

Intermediate System-to-Intermediate System Hello (IIH) PDU — As mentioned previously, the IIH PDU, or Hello packet, detects neighboring routers and indicates to the pseudonode the area’s adjacency mesh. The Hello packet, flooded to the multicast address, contains the system ID of the sending router, the holding time, the circuit type of the interface on which the PDU was sent, the PDU length, the DIS identifier, and the interface priority (used in DIS election). The Hello packet also informs its area routers that it is the DIS.

Hello packets are padded to the maximum IS-IS PDU size of 1492 bytes (the full MTU size) to assist in the detection of transmission errors with large frames or with MTU mismatches between adjacencies.

The DIS typically floods Hello packets to the entire LAN every three seconds.

Link-state PDU (LSP) — The LSP contains information about each router in an area and its connected interfaces. LSPs are refreshed periodically and acknowledged on the network by way of sequence number PDUs. If new LSP information is found, based on the most recent complete sequence number PDU (CSNP), then out-of-date entries in the link-state database (LSDB) are removed and the LSDB is updated.

For a more detailed breakdown of the LSP, see “LSP structure”.

Complete sequence number PDU (CSNP) — CSNPs contain a list of all LSPs in the current LSDB. The CSNP informs other area routers of missing or outdated links in the adjacency mesh. The receiving routers then use this information to update their own database to ensure that all area routers converge.

In contrast to Hello packets, CSNPs are sent every ten seconds and only between neighbor. In other words, they are never flooded.

Partial sequence number PDU (PSNP) — PSNPs are used to request and acknowledge LSP information from an adjacency. When a router compares a CSNP with its local database and determines a discrepancy, the router requests an updated LSP using a PSNP. Once received, the router stores the LSP in its local database and responds to the DIS with acknowledgement.