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By using :manpage:`brtcl(8)`, we can easily observe the state of the Spanning Tree protocol on the different switches. Let us start with ``s3``, i.e. the root of the Spanning Tree.
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The first part of the output of the :manpage:`brctl(8)` command shows the state of the Spanning Tree software on the switch. The identifier of this switch is ``0003.f63545ab5f79`` and the root switch is itself. There is no root port on this switch since it is the root. The path cost is the cost of the path to reach the root switch, i.e. 0 on the root. Then the switch reports the different timers.
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The second part of the output provides the state of each switch port. Port ``s3-eth1`` is active and forwards data frames (state is set to `forwarding`). This port is a `designated` port. The cost of ``1`` is the cost associated to this interface. The same information is found for port ``s3-eth2``.
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The state of switch ``s9`` is different. The output of :manpage:`brctl(8)` indicates that the root identifier is ``0003.f63545ab5f79`` which is at a distance of ``1`` from switch ``s9``. The root port on ``s9`` is port `1`, i.e. ``s9-eth1``. Two of the ports of this switch forward data packets, the root port and the ``s9-eth3`` which is a designated port. The ``s9-eth2`` port is a blocked port.
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:manpage:`brctl(8)` also maintains a MAC address table that contains the Ethernet addresses that have been learned on each switch port.
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Thanks to the traces collected by tcpdump_, we can easily analyze the messages exchanged by the switches. Here is the fist message sent by switch ``s3``.
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