rfc2642.txt

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   Wait Timer

      This event is generated when the one-shot Wait timer expires,
      triggering the end of the required waiting period before the
      switch can begin the process of selecting a designated switch and
      a backup designated switch on a multi-access link.

   Backup Seen

      This event is generated when the switch has detected the existence
      or non-existence of a backup designated switch for the link, as
      determined in one of the following two ways:

      o  A Hello packet has been received from a neighbor that claims to
         be the backup designated switch.

      o  A Hello packet has been received from a neighbor that claims to
         be the designated switch.  In addition, the packet indicated
         that there is no backup.

   In either case, the interface must have bidirectional communication
   with its neighbor -- that is, the local switch must be listed in the
   neighbor's Hello packet.

   This event signals the end of the Waiting state.

   Neighbor change

      This event is generated when there has been one of the following
      changes in the set of bidirectional neighbors associated with the
      interface.  (See Section 4.1 for information on neighbor states.)

      o  Bidirectional communication has been established with a
         neighbor -- the state of the neighbor has changed to 2-Way or
         higher.

      o  Bidirectional communication with a neighbor has been lost --
         the state of the neighbor has changed to Init or lower.



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      o  A bidirectional neighbor has just declared itself to be either
         the designated switch or the backup designated switch, as
         detected by examination of that neighbor's Hello packets.

      o  A bidirectional neighbor is no longer declaring itself to be
         either the designated switch or the backup designated switch,
         as detected by examination of that neighbor's Hello packets.

      o  The advertised switch priority of a bidirectional neighbor has
         changed, as detected by examination of that neighbor's Hello
         packets.

      When this event occurs, the designated switch and the backup
      designated switch must be reselected.

      Loop Ind

         This event is generated when an interface enters the Loopback
         state.  This event can be generated by either the network
         management service or by the lower-level protocols.

      Unloop Ind

         This event is generated when an interface leaves the Loopback
         state.  This event can be generated by either the network
         management service or by the lower-level protocols.

      Interface Down

         This event is generated under the following two circumstances:

         o  The VlanHello [IDhello] protocol has determined that the
            interface is no longer functional.

         o  The neighbor state machine has detected a second neighboring
            switch on a link presumed to be of type point-to-point. In
            addition to generating the Interface Down event, the
            neighbor state machine changes the interface type to
            broadcast.

      In both instances, this event forces the interface state to Down.
      However, when the event is generated by the neighbor state
      machine, it is immediately followed by an Interface Up event.
      (See Section 4.3.)







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3.3 Interface State Machine

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

   Interface states (see Section 3.1) change as the result of various
   events (see Section 3.2).  However, the effect of each event can
   vary, depending on the current state of the interface. For this
   reason, the state machine described in this section is organized
   according to the current interface state and the occurring event.
   For each state/event pair, the new interface state is listed, along
   with a description of the required processing.

   Note that when the state of an interface changes, it may be necessary
   to originate a new switch link advertisement.  See Section 8.1 for
   more information.

   Some of the processing described here includes generating events for
   the neighbor state machine.  For example, when an interface becomes
   inoperative, all neighbor connections associated with the interface
   must be destroyed.  For more information on the neighbor state
   machine, see Section 4.3.

   State(s):  Down
   Event:  Interface Up
   New state:  Depends on action routine
   Action:
      If the interface is a point-to-point link, set the interface state
      to Point-to-Point.  Otherwise, start the Hello interval timer,
      enabling the periodic sending of Hello packets over the interface.
      If the switch is not eligible to become the designated switch,
      change the interface state to DS Other. Otherwise, set the
      interface state to Waiting and start the one-shot wait timer.
      Create a new neighbor data structure for the neighbor switch,
      initialize all neighbor parameters and set the stateof the
      neighbor to Down.

   State(s):  Waiting
   Event:  Backup Seen
   New state:  Depends on action routine
   Action:
      Select the designated switch and backup designated switch for the
      attached link, as described in Section 6.3.1.  As a result of this
      selection, set the new state of the interface to either DS Other,
      Backup or DS.






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   State(s):  Waiting
   Event:  Wait Timer
   New state:  Depends on action routine
   Action:
      Select the designated switch and backup designated switch for the
      attached link, as described in Section 6.3.1.  As a result of this
      selection, set the new state of the interface to either DS Other,
      Backup or DS.

   State(s):  DS Other, Backup or DS
   Event:  Neighbor Change
   New state:  Depends on action routine
   Action:
      Reselect the designated switch and backup designated switch for
      the attached link, as described in Section 6.3.1.  As a result of
      this selection, set the new state of the interface to either DS
      Other, Backup or DS.

   State(s):  Any State
   Event:  Interface Down
   New state:  Down
   Action:
      Reset all variables in the interface data structure and disable
      all timers.  In addition, destroy all neighbor connections
      associated with the interface by generating the KillNbr event on
      all neighbors listed in the interface data structure.

   State(s):  Any State
   Event:  Loop Ind
   New state:  Loopback
   Action:
      Reset all variables in the interface data structure and disable
      all timers.  In addition, destroy all neighbor connections
      associated with the interface by generating the KillNbr event on
      all neighbors listed in the interface data structure.

   State(s):  Loopback
   Event:  Unloop Ind
   New state:  Down
   Action:
      No action is necessary beyond changing the interface state to Down
      because the interface was reset on entering the Loopback state.









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4. Neighbor Data Structure

   Each switch conducts a conversation with its neighboring switches and
   each conversation is described by a neighbor data structure.  A
   conversation is associated with a switch interface, and is identified
   by the neighboring switch ID.

   Note that if two switches have multiple attached links in common,
   multiple conversations ensue, each described by a unique neighbor
   data structure.  Each separate conversation is treated as a separate
   neighbor.

   The neighbor data structure contains all information relevant to any
   adjacency formed between the two neighbors.  Remember, however, that
   not all neighbors become adjacent.  An adjacency can be thought of as
   a highly developed conversation between two switches.

   State

      The functional level of the neighbor conversation.  See Section
      4.1 for a complete description of neighbor states.

   Inactivity timer

      A one-shot timer used to determine when to declare the neighbor
      down if no Hello packet is received from this (multi-access)
      neighbor.  The length of the timer is SwitchDeadInterval seconds,
      as contained in the neighbor's Hello packet.  This timer is not
      used on point-to-point links.

   Master/slave flag

      A flag indicating whether the local switch is to act as the master
      or the slave in the database exchange process (see Section 7.2).
      The master/slave relationship is negotiated when the conversation
      changes to the ExStart state.

   Sequence number

      A 4-octet number identifying individual Database Description
      packets. When the neighbor state ExStart is entered and the
      database exchange process is started, the sequence number is set
      to a value not previously seen by the neighboring switch. (One
      possible scheme is to use the switch's time of day counter.)  The
      sequence number is then incremented by the master with each new
      Database Description packet sent.  See Section 7.2 for more
      information on the database exchange process.




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   Neighbor ID

      The switch ID of the neighboring switch, as discovered by the
      VlanHello protocol [IDhello] or contained in the neighbor's Hello
      packets.

   Neighbor priority

      The switch priority of the neighboring switch, as contained in the
      neighbor's Hello packets.  Switch priorities are used when
      selecting the designated switch for the attached multi-access
      link.  Priority is not used on point-to-point links.

   Interface identifier

      A 10-octet value that uniquely identifies the interface over which
      this conversation is being held.  This value consists of the 6-
      octet base MAC address of the neighbor switch, followed by the 4-
      octet local port number of the interface.

   Neighbor's designated switch

      The switch ID identifying the neighbor's idea of the designated
      switch, as contained in the neighbor's Hello packets.  This value
      is used in the local selection of the designated switch.  It is
      not used on point-to-point links.

   Neighbor's backup designated switch

      The switch ID identifying the neighbor's idea of the backup
      designated switch, as contained in the neighbor's Hello packets.
      This value is used in the local selection of the backup designated
      switch.  It is not used on point-to-point links.

   Link state retransmission list

      The list of link state advertisements that have been forwarded
      over but not acknowledged on this adjacency.  The local switch
      retransmits these link state advertisements at periodic intervals
      until they are acknowledged or until the adjacency is destroyed.
      (For more information on retransmitting link state advertisements,
      see Section 8.2.5.)









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   Database summary list

      The set of link state advertisement headers that summarize the
      local link state database.  When the conversation changes to the
      Exchange state, this list is sent to the neighbor via Database
      Description packets.  (For more information on the synchronization
      of databases, see Section 7.)

   Link state request list

      The list of link state advertisements that must be received in
      order to synchronize with the neighbor switch's link state
      database.  This list is created as Database Description packets
      are received, and is then sent to the neighbor in Link State