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The ``addSwitch`` method creates an Ethernet switch. It assigns a random MAC address to each switch and we can configure it with a priority that is used in the high order bits of the switch identifier. We add one IP address to each switch so that we can connect to them on mininet_. In practice, IPMininet_ configures the :manpage:`brtcl(8)` software that implements the Spanning Tree protocol on Linux. We can then create the links, configure their cost if required and launch tcpdump_ to capture the Ethernet frames that contain the messages of the Spanning Tree protocol.
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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 network contains five nodes and six links.
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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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This network can be launched with the IPMininet_ script shown below. The entire script is available from :download:`/exercises/ipmininet_scripts/stp.py`.
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