Message Types OSPF/IS-IS

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, there are nine actual types:

  • Level 1 LAN IIH
  • Level 2 LAN IIH
  • Point-to-Point IIH
  • Level 1 CSNP
  • Level 2 CSNP
  • Level 1 PSNP
  • Level 2 PSNP
  • Level 1 LSP
  • Level 2 LSP

Hello messages serve the same three purposes in both OSPF and IS-IS. They are used to discover neighbors, to negotiate adjacencies, and, on established adjacencies, serve as keepalives. Although IS-IS Hellos are officially assigned the acronym IIH, as shown in the previous two lists, in this book I use the term Hello to refer to both OSPF and IS-IS Hellos for the sake of simplicity.

OSPF Database Description, Link State Request, and Link State Acknowledgement packets are used for the OSPF database synchronization process. Similarly, IS-IS sequence number PDUs are used for the IS-IS database synchronization process. The formats of these messages, and a comparison of their uses,

Table 3.2 associates the OSPF and IS-IS message types by function. The most interesting comparison in the table is that between OSPF Updates and IS-IS LSPs. Earlier in the section on comparative terminology, I equated LSPs to LSAs, yet here I equate LSPs to Updates. This seeming inconsistency is at the heart of the difference between LSAs and LSPs, and is explained in the following section.

Table 3.2. A Comparison of OSPF and IS-IS Messages by Function
Table 3.2





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