rfc2642.txt

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      Request packets.  (For more information on the synchronization of
      databases, see Section 7.)

4.1 Neighbor States

   This section describes the various states of a conversation with a
   neighbor switch.  The states are listed in order of progressing
   functionality.  For example, the inoperative state is listed first,
   followed by a list of the intermediate states through which the
   conversation passes before attaining the final, fully functional
   state.  The specification makes use of this ordering by references
   such as "those neighbors/adjacencies in state greater than X".

   Figure 2 represents the neighbor state machine.  The arrows on the
   graph represent the events causing each state change.  These events
   are described in Section 4.2.  The neighbor state machine is
   described in detail in Section 4.3.

   Down

      This is the initial state of a neighbor conversation.

   Init

      In this state, the neighbor has been discovered, but bidirectional
      communication has not yet been established. All neighbors in this
      state or higher are listed in the VLS Hello packets sent by the
      local switch over the associated (multi-access) interface.









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          +----------+     KillNbr, LLDown,   +-----------+
          |   Down   | <--------------------- | any state |
          +----------+   or Inactivity Timer  +-----------+
               |
         Hello |
          Rcvd |
               |
               V
   +-----< [pt-to-pt?]
   | yes       |
   |           | no
   |           V
   |      +----------+   1-Way   +----------+
   |      |   Init   | <-------- | >= 2-way |
   |      +----------+           +----------+
   |           |
   |     2-Way |
   |      Rcvd |                  +-------+   AdjOK? +------------+
   |           +----------------> | 2-Way | <------- | >= ExStart |
   |           | (no adjacency)   +-------+     no   +------------+
   |           |
   |           V
   |      +---------+   Seq Number Mismatch  +-------------+
   +----> | ExStart | <--------------------- | >= Exchange |
          +---------+       or BadLSReq      +-------------+
               |
   Negotiation |
       Done    |
               V
          +----------+
          | Exchange |
          +----------+
               |
      Exchange |                        +--------+
        Done   +----------------------> |  Full  |
               | (request list empty)   +--------+
               |                             ^
               V                             |
          +---------+      Loading Done      |
          | Loading | ----------------------->
          +---------+

                  Figure 2: Neighbor State Machine








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   2-Way

      In this state, communication between the two switches is
      bidirectional.  This is the most advanced state short of beginning
      to establish an adjacency.  On a multi-access link, the designated
      switch and the backup designated switch are selected from the set
      of neighbors in state 2-Way or greater.

   ExStart

      This state indicates that the two switches have begun to establish
      an adjacency by determining which switch is the master, as well as
      the initial sequence number for Database Descriptor packets.
      Neighbor conversations in this state or greater are called
      adjacencies.

   Exchange

      In this state, the switches are exchanging Database Description
      packets.  (See Section 7.2 for a complete description of this
      process.)  All adjacencies in the Exchange state or greater are
      used by the distribution procedure (see Section 8.2), and are
      capable of transmitting and receiving all types of VLSP routing
      packets.

   Loading

      In this state, the local switch is sending Link State Request
      packets to the neighbor asking for the more recent advertisements
      that were discovered in the Exchange state.

   Full

      In this state, the two switches are fully adjacent.  These
      adjacencies will now appear in switch link and network link
      advertisements generated for the link.

4.2 Events Causing Neighbor State Changes

   The state of a neighbor conversation changes due to neighbor events.
   This section describes these events.

   Neighbor events are shown as arrows in Figure 2, the graphic
   representation of the neighbor state machine.  For more information
   on the neighbor state machine, see Section 4.3.






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   Hello Received

      This event is generated when a Hello packet has been received from
      a neighbor.

   2-Way Received

      This event is generated when the local switch sees its own switch
      ID listed in the neighbor's Hello packet, indicating that
      bidirectional communication has been established between the two
      switches.

   Negotiation Done

      This event is generated when the master/slave relationship has
      been successfully negotiated and initial packet sequence numbers
      have been exchanged.  This event signals the start of the database
      exchange process (see Section 7.2).

   Exchange Done

      This event is generated when the database exchange process is
      complete and both switches have successfully transmitted a full
      sequence of Database Description packets.  (For more information
      on the database exchange process, see Section 7.2.)

   BadLSReq

      This event is generated when a Link State Request has been
      received for a link state advertisement that is not contained in
      the database.  This event indicates an error in the
      synchronization process.

   Loading Done

      This event is generated when all Link State Updates have been
      received for all out-of-date portions of the database.  (See
      Section 7.3.)

   AdjOK?

      This event is generated when a decision must be made as to whether
      an adjacency will be established or maintained with the neighbor.
      This event will initiate some adjacencies and destroy others.







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   Seq Number Mismatch

      This event is generated when a Database Description packet has
      been received with any of the following conditions:

      o  The packet contains an unexpected sequence number.
      o  The packet (unexpectedly) has the Init bit set.
      o  The packet has a different Options field than was
         previously seen.

      These conditions all indicate that an error has occurred during
      the establishment of the adjacency.

   1-Way

      This event is generated when bidirectional communication with the
      neighbor has been lost.  That is, a Hello packet has been received
      from the neighbor in which the local switch is not listed.

   KillNbr

      This event is generated when further communication with the
      neighbor is impossible.

   Inactivity Timer

      This event is generated when the inactivity timer has expired,
      indicating that no Hello packets have been received from the
      neighbor in SwitchDeadInterval seconds.  This timer is used only
      on broadcast (multi-access) links.

   LLDown

      This event is generated by the lower-level switch discovery
      protocols and indicates that the neighbor is now unreachable.

4.3 Neighbor State Machine

   This section presents a detailed description of the neighbor state
   machine.

   Neighbor states (see Section 4.1) change as the result of various
   events (see Section 4.2).  However, the effect of each event can
   vary, depending on the current state of the conversation with the
   neighbor.  For this reason, the state machine described in this
   section is organized according to the current neighbor state and the
   occurring event.  For each state/event pair, the new neighbor state
   is listed, along with a description of the required processing.



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   Note that when the neighbor state changes as a result of an interface
   Neighbor Change event (see Section 3.2), it may be necessary to rerun
   the designated switch selection algorithm. In addition, if the
   interface associated with the neighbor conversation is in the DS
   state (that is, the local switch is the designated switch), changes
   in the neighbor state may cause a new network link advertisement to
   be originated (see Section 8.1).

   When the neighbor state machine must invoke the interface state
   machine, it is invoked as a scheduled task.  This simplifies
   processing, by ensuring that neither state machine executes
   recursively.

   State(s):  Down
   Event:  Hello Received
   New state:  Depends on the interface type
   Action:
      If the interface type of the associated link is point-to-point,
      change the neighbor state to ExStart.  Otherwise, change the
      neighbor state to Init and start the inactivity timer for the
      neighbor.  If the timer expires before another Hello packet is
      received, the neighbor switch is declared dead.

   State(s):  Init or greater
   Event:  Hello Received
   New state:  No state change
   Action:
      If the interface type of the associated link is point-to-point,
      determine whether this notification is for a different neighbor
      than the one previously seen. If so, generate an Interface Down
      event for the associated interface, reset the interface type to
      broadcast and generate an Interface Up event.

   Note:  This procedure of generating an Interface Down event and
   changing the interface type to broadcast is also executed if the
   neighbor for whom the notification was received is running an older
   version of the protocol software (see Section 6.1).  In previous
   versions of the protocol, all interfaces were treated as if they were
   broadcast.

      If the interface type is broadcast, reset the inactivity timer for
      the neighbor.









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   State(s):  Init
   Event:  2-Way Received
   New state:  Depends on action routine
   Action:
      Determine whether an adjacency will be formed with the neighbor
      (see Section 6.4).  If no adjacency is to be formed, change the
      neighbor state to 2-Way.

      Otherwise, change the neighbor state to ExStart.  Initialize the
      sequence number for this neighbor and declare the local switch to
      be master for the database exchange process.  (See Section 7.2.)

   State(s):  ExStart
   Event:  Negotiation Done
   New state:  Exchange