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I'm following the Cisco routing & switching course, but there is something I don't understand. It is said that OSPF exchange five packet type:

  • Hello (discover neighbors)
  • DBD (database description)
  • LSR (link state request)
  • LSU (link state update)
  • LS ack (link state acknowledges)

I understand what these packets do, but when there is LSA (link state advertisement). As Cisco say, LSA are used to create/maintain the link state database. So what is the difference between LSA and LSR/LSU/LSAck ? As I understand, they all are used for the exact same purpose.

Thank you.

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  • LSA

Link state advertisement It is a message that communicates the router's local routing topology to all other local routers in the same OSPF area. This LSA has types depend on the type of router and has also sequence number.

  • LSDB

Every OSPF router maintains a Link state database (LSDB). Each router stores the received LSA packets in the link-state database (LSDB). After LSDBs are synced between the routers, OSPF uses the shortest path first (SPF) algorithm to calculate the best routes. (full version of the database)

  • DBDs

Database description packets (also referred as DDPs) These packets are exchanged when an adjacency is being initialized. They describe the contents of the topological database. It does not include full LSAs but would include LSA headers in the link-state database of the sender..

  • LSR

After exchanging Database Description packets with a neighboring router, a router may find that parts of its topological database are out of date. The Link State Request packet is used to request the pieces of the neighbor's database that are more up to date. The sending of Link State Request packets is the last step in bringing up an adjacency What other have (DBDs) – What I have (LADB) = What I need to order (LSR)

  • Link-State Update (LSU)

A packet that contains fully detailed LSAs, typically sent in response to an LSR message

  • Link-State Acknowledgment (LSAck)

Sent to confirm receipt of an LSU message

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OSPF uses link-state packets (LSPs) to establish and maintain neighbor adjacencies and exchange routing updates. There are 5 different LSP types and you have just mentioned them in your question. What's important to understand, is that LSU (link state update packets) can CONTAIN 11 different types of OSPF LSAs. The key point to remember is that the difference between LSU and LSA terms can sometimes be confusing because these terms are often used interchangeably. Hovewer, an LSU contains one or more LSAs.

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LSA can be seen as "item of information": Link State Advertisement. LSU,LSR, LSack are messages managing the sync between routers of this items.

LSU: link state update LSR: link state request LSack: link state ack

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