OSPF and IS-IS: Choosing an IGP for Large-Scale Networks

3.4. Message Types

OSPF uses five message types:

• Hello

• Database Description (DD)

• Link State Request

• Link State Acknowledgement

• Link State Update

IS-IS uses four basic message types:

• IS-IS Hello (IIH)

• Complete Sequence Number PDU (CSNP)

• Partial Sequence Number PDU (PSNP)

• Link State PDU (LSP)

Unlike OSPF, IS-IS messages have subtypes. There are LAN and Point-to-Point Hellos, used as the names imply on either broadcast or point-to-point media. The LAN Hellos are also subdivided into level 1 and level 2 types and are sent over level 1 and level 2 adjacencies. Likewise, Sequence Number PDUs (CSNPs and PSNPs) and LSPs are also subdivided into level 1 and level 2 types. So although there are only four basic types of IS-IS messages, when divided by function, ...