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

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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, ...

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