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9.3. The Interface state machine Connected: An Internet Encyclopedia
9.3. The Interface state machine

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Up: 9. The Interface Data Structure
Prev: 9.2. Events causing interface state changes
Next: 9.4. Electing the Designated Router

9.3. The Interface state machine

9.3. The Interface state machine

A detailed description of the interface state changes follows. Each state change is invoked by an event (Section 9.2). This event may produce different effects, depending on the current state of the interface. For this reason, the state machine below is organized by current interface state and received event. Each entry in the state machine describes the resulting new interface state and the required set of additional actions.

When an interface's state changes, it may be necessary to originate a new router links advertisement. See Section 12.4 for more details.

Some of the required actions below involve 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 10.3.

         State(s):  Down

            Event:  InterfaceUp

        New state:  Depends upon action routine

           Action:  Start the interval Hello Timer, enabling the
                    periodic sending of Hello packets out the interface.
                    If the attached network is a physical point-to-point
                    network or virtual link, the interface state
                    transitions to Point-to-Point.  Else, if the router
                    is not eligible to become Designated Router the
                    interface state transitions to DR Other.

                    Otherwise, the attached network is multi-access and
                    the router is eligible to become Designated Router.
                    In this case, in an attempt to discover the attached
                    network's Designated Router the interface state is
                    set to Waiting and the single shot Wait Timer is
                    started.  If in addition the attached network is
                    non-broadcast, examine the configured list of
                    neighbors for this interface and generate the
                    neighbor event Start for each neighbor that is also
                    eligible to become Designated Router.

         State(s):  Waiting

            Event:  BackupSeen

        New state:  Depends upon action routine.

           Action:  Calculate the attached network's Backup Designated
                    Router and Designated Router, as shown in Section
                    9.4.  As a result of this calculation, the new state
                    of the interface will be either DR Other, Backup or
                    DR.

         State(s):  Waiting

            Event:  WaitTimer

        New state:  Depends upon action routine.

           Action:  Calculate the attached network's Backup Designated
                    Router and Designated Router, as shown in Section
                    9.4.  As a result of this calculation, the new state
                    of the interface will be either DR Other, Backup or
                    DR.

         State(s):  DR Other, Backup or DR

            Event:  NeighborChange

        New state:  Depends upon action routine.

           Action:  Recalculate the attached network's Backup Designated
                    Router and Designated Router, as shown in Section
                    9.4.  As a result of this calculation, the new state
                    of the interface will be either DR Other, Backup or
                    DR.

         State(s):  Any State

            Event:  InterfaceDown

        New state:  Down

           Action:  All interface variables are reset, and interface
                    timers disabled.  Also, all neighbor connections
                    associated with the interface are destroyed.  This
                    is done by generating the event KillNbr on all
                    associated neighbors (see Section 10.2).

         State(s):  Any State

            Event:  LoopInd

        New state:  Loopback

           Action:  Since this interface is no longer connected to the
                    attached network the actions associated with the
                    above InterfaceDown event are executed.

         State(s):  Loopback

            Event:  UnloopInd

        New state:  Down

           Action:  No actions are necessary.  For example, the
                    interface variables have already been reset upon
                    entering the Loopback state.  Note that reception of
                    an InterfaceUp event is necessary before the
                    interface again becomes fully functional.


Next: 9.4. Electing the Designated Router

Connected: An Internet Encyclopedia
9.3. The Interface state machine

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