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Detecting and handling a better configuration

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• When is a new configuration better better than the current configuration
  • There are 2 cases that can improve the current configuration:

    The root bridge ID in the new configuration is smaller: We have found a better root bridge !!!       (Since the root bridge is the bridge with the smallest ID, the current root brigde is actually wrong !!!) A: The root bridge ID is equal to the current root bridge ID       and        B: The distance to the root bridge is smaller than the current distance: We have found a shorter route to the (same) root bridge !!!

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• How to determine a better root bridge
  • Test to find a better root bridge

    Root ID of the control message    <    MyRoot

    Example:

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  • Actions taken by the bridge when a better root bridge is discovered:

    MyRoot = Root Bridge ID in configuration message; Distance = Distance in configuration message + 1; // One more hop ! State(Port on which configuration message was recv) = R (Root); for ( all other ports P ) { State(P) = D (Designated); Transmit configuration msg "(myID, MyRoot, Distance)" on port P; }

    Example:

    Bridge 5 has just received a configuration message with MyRoot in configuration msg < Current Root Scenario: Bridge 5 will update its state variables to: and transmit the following configuration message on all its other ports:

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• How to determine a shorter path to the same root bridge
  • Test used to find a shorter path to the (same) root bridge:

    (Root ID in control message == my Root)        and       (Distance in control message + 1     <     my Distance)

    Example:

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  • Actions taken by the bridge when a better root bridge is discovered:

    ( MyRoot = Root Bridge ID in configuration message; ) // They are equal !!! Distance = Distance in configuration message + 1; // One more hop ! State(Port on which configuration message was recv) = R (Root); for ( all other ports P ) { State(P) = D (Designated); Transmit configuration msg "(myID, MyRoot, Distance)" on port P; }

    Example:

    Bridge 5 has just received a configuration message with MyRoot in configuration msg = Current Root and Distance in configuration msg < Current Distance Scenario: Bridge 5 will update its state variables to: and transmit the following configuration message on all its other ports: