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Many OSPF sources mention that during the EXSTART state, routers communicate and decide who will be the master and slave based on their RIDs (the router with the higher RID becomes the master).

However, don't they already know their RIDs at this point? They had already confirmed 2-way reachability using hello packets that also contained their RID. So why not just calculate who the master and slave is based off that and jump straight into the EXCHANGE state?

Thank you in advance for your help.

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  • There is more to it than that. It is not only deciding which is the master, but also the initial Database Descriptor sequence number and sending Initial DD packets until the Exchange state is achieved. The state machine is detailed in the RFC.
    – Ron Maupin
    May 20, 2023 at 14:08

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Quoting RFC2328:

"This is the first step in creating an adjacency between the two neighboring routers. The goal of this step is to decide which router is the master, and to decide upon the initial DD sequence number. Neighbor conversations in this state or greater are called adjacencies."

Note that MTU in those headers needs to match otherwise the routers will be stuck in this phase. To break down the process with preceding and following steps:

  • 2-WAY completed after Hello packets are seen
  • EXSTART establishes who is the master-slave with initial DBDs sent and received
  • EXCHANGE master initiates exchange of DBDs with LSAs, once exchange is completed routers move to next stage. In other words - routers describe their databases to each other
  • LOADING requesting and sending information about any LSAs that are missing on a router. The actual missing contents of the database are populated.
  • FULL all information exchanged and routes can make their way to the routing table

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