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Dealing with loops --- physical network and logical network

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• Loop-free networks
  • A loop-free network is:

    a tree

    Example:

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• Logical and Physical networks
  • Physical network:

    Physical network = the network that consists of the actual nodes and links Example:

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  • Logical network:

    Logical network = the (sub)network that consists of the nodes and links that are used to forward frames Example:

    Note:

    We must make sure that the logical network is a tree !!!

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• How to provide fault tolerance
  • Fault-tolerant networking:

    The physical network contains loops This is necessary to provide alternative routes We create a loop-free logical network (= tree) on top of the physical network by: Disabling one or more ports on switches that is part of a loop           Frame forwarding will only use the logical network

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• The IEEE 802.1D "Spanning Tree" algorithm

  • Next:

    We will study a distributed algorithm that the Ethernet switches execute to: Form a tree (= the logical network)           The algorithm is called the IEEE 8021D Spanning Tree Algorithm