rfc2642.txt

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   o  Section 3 through Section 5 describe the primary data structures
      used by the protocol.  Note that this specification is presented
      in terms of these data structures in order to make explanations
      more precise.  Implementations of the protocol must support the
      functionality described, but need not use the exact data
      structures that appear in this specification.

   o  Section 6 through Section 9 describe the four operational stages
      of the protocol:  the discovery process, synchronizing the
      databases, maintaining the databases, and calculating the set of
      best paths.

   o  Section 10 describes the processing of VLSP packets and presents
      detailed descriptions of their formats.

   o  Section 11 presents detailed descriptions of link state
      advertisements.

   o  Section 12 summarizes the protocol parameters.






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RFC 2642         Cabletron's VLS Protocol Specification      August 1999


3. Interface Data Structure

   The port over which a switch accesses a network link is known as the
   link interface.  Each switch maintains a separate interface data
   structure for each network link.

   The following data items are associated with each interface:

   Type

      The type of network to which the interface is attached -- point-
      to-point or broadcast (multi-access).  This data item is
      initialized to point-to-point when the interface becomes
      operational.  If a second neighbor is detected on the link after
      the first neighbor has been discovered, the link interface type is
      changed to broadcast.  The type remains as broadcast until the
      interface is declared down, at which time the type reverts to
      point-to-point.

   Note:  Previous versions of VLSP treated all links as if they were
   multi-access.  Thus, if VLSP determines that a neighbor switch is
   running an older version of the protocol software (see Section 6.1),
   it will change the interface type to broadcast.

   State

      The functional level of the interface.  The state of the interface
      is included in all switch link advertisements generated by the
      switch, and is also used to determine whether full adjacencies are
      allowed on the interface.  See Section 3.1 for a complete
      description of interface states.

   Interface identifier

      A 10-octet value that uniquely identifies the interface. 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.

   Area ID

      A 4-octet value identifying the area.  Since VLSP does not support
      multiple areas, the value here is always zero.








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   HelloInterval

      The interval, in seconds, at which the switch sends VLSP Hello
      packets over the interface.  This parameter is not used on point-
      to-point links.

   SwitchDeadInterval

      The length of time, in seconds, that neighboring switches will
      wait before declaring the local switch dNeighboring switches

      A list of the neighboring switches attached to this network link.
      This list is created during the discovery process. Adjacencies are
      formed to one or more of these neighbors. The set of adjacent
      neighbors can be determined by examining the states of the
      neighboring switches as shown in their link state advertisements.

   Designated switch

      The designated switch selected for the multi-access network link.
      (A designated switch is not selected for a point-to-point link.)
      This data item is initialized to zero when the switch comes on-
      line, indicating that no designated switch has been chosen for the
      link.

   Backup designated switch

      The backup designated switch selected for the multi-access network
      link.  (A backup designated switch is not selected for a point-
      to-point link.)  This data item is initialized to zero when the
      switch comes on-line, indicating that no backup designated switch
      has been chosen for the link.

   Interface output cost(s)

      The cost of sending a packet over the interface.  The link cost is
      expressed in the link state metric and must be greater than zero.

   RxmtInterval

      The number of seconds between link state advertisement
      retransmissions, for adjacencies belonging to this interface. This
      value is also used to time the retransmission of Database
      Description and Link State Request packets.







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RFC 2642         Cabletron's VLS Protocol Specification      August 1999


3.1 Interface States

   This section describes the various states of a switch interface. 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 interface passes before
   attaining the final, fully functional state.  The specification makes
   use of this ordering by references such as "those interfaces in state
   greater than X".

   Figure 1 represents the interface state machine, showing the
   progression of interface state changes.  The arrows on the graph
   represent the events causing each state change.  These events are
   described in Section 3.2.  The interface state machine is described
   in detail in Section 3.3.

   Down

      This is the initial state of the interface.  In this state, the
      interface is unusable, and no protocol traffic is sent or received
      on the interface.  In this state, interface parameters are set to
      their initial values, all interface timers are disabled, and no
      adjacencies are associated with the interface.




























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RFC 2642         Cabletron's VLS Protocol Specification      August 1999


       +-------+
       |  any  |  Interface   +----------+  Unloop Ind  +----------+
       | state | -----------> |   Down   | <----------- | Loopback |
       +-------+    Down      +----------+              +----------+
                                   |                         ^
                                   | Interface Up            |
           +-------+  [pt-to-pt]   |                         |
           | Point |<------------type?              Loop Ind |
           |  to   |               |                         |
           | Point |               | [broadcast]             |
           +-------+               V                     +-------+
                             +-----------+               |  any  |
                             |  Waiting  |               | state |
                             +-----------+               +-------+
                                   |
                       Backup Seen |
                                   | Wait Timer
                                   |
                                   |
      +----------+    Neighbor     V     Neighbor    +----------+
      |    DS    | <------------> [ ] <------------> | DS Other |
      +----------+     Change      ^      Change     +----------+
                                   |
                                   |
                   Neighbor Change |
                                   |
                                   V
                              +----------+
                              |  Backup  |
                              +----------+

                   Figure 1:  Interface State Machine


   Loopback

      In this state, the switch interface is looped back, either in
      hardware or in software.  The interface is unavailable for regular
      data traffic.

   Point-to-Point

      In this state, the interface is operational and is connected to a
      physical point-to-point link.  On entering this state, the switch
      attempts to form an adjacency with the neighboring switch.






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RFC 2642         Cabletron's VLS Protocol Specification      August 1999


   Waiting

      In this state, the switch is attempting to identify the backup
      designated switch for the link by monitoring the Hello packets it
      receives.  The switch does not attempt to select a designated
      switch or a backup designated switch until it changes out of this
      state, thereby preventing unnecessary changes of the designated
      switch and its backup.

   DS Other

      In this state, the interface is operational and is connected to a
      multi-access broadcast link on which other switches have been
      selected as the designated switch and the backup designated
      switch.   On entering this state, the switch attempts to form
      adjacencies with both the designated switch and the backup
      designated switch.

   Backup

      In this state, the switch itself is the backup designated switch
      on the attached multi-access broadcast link.  It will be promoted
      to designated switch if the current designated switch fails.  The
      switch establishes adjacencies with all other switches attached to
      the link.  (See Section 6.3 for more information on the functions
      performed by the backup designated switch.)

   DS

      In this state, this switch itself is the designated switch on the
      attached multi-access broadcast link.  The switch establishes
      adjacencies with all other switches attached to the link.  The
      switch is responsible for originating network link advertisements
      for the link, containing link information for all switches
      attached to the link, including the designated switch itself.
      (See Section 6.3 for more information on the functions performed
      by the designated switch.)

3.2 Events Causing Interface State Changes

   The state of an interface changes due to an interface event.  This
   section describes these events.

   Interface events are shown as arrows in Figure 1, the graphic
   representation of the interface state machine.  For more information
   on the interface state machine, see Section 3.3.





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RFC 2642         Cabletron's VLS Protocol Specification      August 1999


   Interface Up

      This event is generated by the VlanHello protocol [IDhello] when
      it discovers a neighbor switch on the interface.  The interface is
      now operational.  This event causes the interface to change out of
      the Down state.  The state it enters is determined by the
      interface type.  If the interface type is broadcast (multi-
      access), this event also causes the switch to begin sending
      periodic Hello packets out over the interface.

   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.)