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The OSPF ExStart state

  • 1.  The OSPF ExStart state

     
    Posted 02-28-2021 02:25
    Hi everyone.

    This is a very academic question.

    The OSPF ExStart state is meant to decide which of the two routers in an OSPF adjacency with be the 'master' in the exchange of databases. The OSPF router with the higher Router ID will be elected as the master. 

    But the routers already know each others Router IDs, via the Hello messages they exchanged to reach the ExStart state. The have the information to already know who the master should be. Why do they go through an unnecessary election in the ExStart state. (In other words, why can't we jump straight from the 2-Way to the Exchange state).

    Thanks,
    Deepak


  • 2.  RE: The OSPF ExStart state

    Posted 03-01-2021 12:40
    There are other ways you could answer this, but here's one.

    OSPF control packets are not all the same, there are different types.  Each type has different fields that tells the other OSPF router what to expect.  Here's an excerpt from the RFC:

                              Type   Description
                              ________________________________
                              1      Hello
                              2      Database Description
                              3      Link State Request
                              4      Link State Update
                              5      Link State Acknowledgment

    > The OSPF ExStart state is meant to decide which of the two routers in an OSPF adjacency with be the 'master' in the exchange of databases. The OSPF router with the higher Router ID will be elected as the master. 

    ExStart does more than just elect master/slave, it also determines the initial sequence number for DD exchange and handles MTU checks. 

    > But the routers already know each others Router IDs, via the Hello messages they exchanged to reach the ExStart state.

    Again, you can't negotiate all of the requirements with with just a Router ID.  Type-2 OSPF packets (DD) contain the Interface MTU, I/M/MS bits, and the LSA headers themselves.  Type-1 (Hello) packets do not have these.

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    Jordan Head
    Resident Engineer - Sr. Staff
    JNCIEx3 (SP|DC|ENT)
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