- The (Distributed) Spanning Tree Algorithm
in
pseudo code:
Let: msg = control message received p = port on which the message msg was received /* =========================================== Check for better root =========================================== */ if ( msg.rootID < bridge.rootID ) { /* ===================================== Update state variables ===================================== */ bridge.rootID = msg.rootID; bridge.distance = msg.distance + 1; status[p] = R; // This is the root port for ( all ports q ≠ incoming port p ) { status[q] = D; // Other ports are now designated } /* ===================================== Forward new state to neighbors ===================================== */ for ( all ports q ≠ incoming port p ) { send (bridge.ID, bridge.rootID, bridge.distance) on port q; } } /* =========================================== Check for shorter path to root bridge =========================================== */ else if ( msg.rootID == bridge.rootID && msg.distance + 1 < bridge.distance ) { /* ===================================== Update state variables ===================================== */ bridge.rootID = msg.rootID; bridge.distance = msg.distance + 1; status[p] = R; // This is the root port for ( all ports q ≠ incoming port p ) { status[q] = D; // Other ports are now designated } /* ===================================== Forward new state to neighbors ===================================== */ for ( all ports q ≠ incoming port p ) { send (bridge.ID, bridge.rootID, bridge.distance) on port q; } } /* =========================================== Check for farthest node in even cycle =========================================== */ else if ( msg.rootID == bridge.rootID && msg.distance + 1 == bridge.distance ) { if ( status[p] != R ) status[p] = B; // Block the incoming port } /* =========================================== Check for farthest node in odd cycle =========================================== */ else if ( msg.rootID == bridge.rootID && msg.distance == bridge.distance ) { if ( msg.ID < bridge.ID ) status[p] = B; // Block the incoming port } else { // Do nothing, ignore a worse configuration }
- Consider the following
interconnect LANs:
- Initialization
of the IEEE 802.1D Spanning Tree
Algorithm
(at time of start up):
- Each bridge assume that it is the the root bridge
Result:
- Recall:
- The Root bridge x will
periodically transmit
the configuration message:
( x, x, 0 )
- The Root bridge x will
periodically transmit
the configuration message:
- Start
of message exchange:
- Messages that are sent:
- Computations performed by
each node:
Explanation:
- Bridge 1 and
bridge 3 found a
better root bridge 0
- Set incoming port to root
- Set other ports to designated
- Bridge 2 found a
better root bridge 1
- Set incoming port to root
- Set other ports to designated
- Bridge 1 and
bridge 3 found a
better root bridge 0
- State after
the computations:
- Messages that are sent:
- Next,
bridges that computed a
better configuration will
forward a
configuration message on its
designated port:
- Messages that
are sent:
- Computations performed by
each node:
Explanation:
- Bridge 1 detects an
odd cycle
- However, since the ID (=3) in the message is larger than the bridge (= 1), bridge 1 will do nothing
- Bridge 3
also detects an
odd cycle
- Because, since the ID = 1 in the message is smaller than the bridge (3), bridge 3 will change the incoming port to blocked
- Bridge 2 found a
better root bridge 1
- Set incoming port to root
- Set other ports to designated
- Bridge 1 detects an
odd cycle
- State
after the computations:
- Messages that
are sent:
- Again,
bridges that computed a
better configuration will
forward a
configuration message on its
designated port:
- Messages that
are sent:
- No computations are performed....
DONE !!!
- Messages that
are sent:
- Final configuration:
Observed that:
- Because port#2 of
Bridge 3
is blocked:
- The loop in the physical newtork is broken
- The learning algorithm
of the switches
(see: click here
),
the forwarding database (table)
will
now
be constructed correctly...
- Recall that the learning technique failed because of loops.....
(The learning bridge technique will work again)
- Because port#2 of
Bridge 3
is blocked: