7.2.2 This page will explain how STP can be used to create a loop free network.
Ethernet bridges and switches can implement the IEEE 802.1d Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network.
Shortest path is based on cumulative link costs. Link costs are based on the speed of the link.
The Spanning-Tree Protocol establishes a root node called the root bridge. The Spanning-Tree Protocol constructs a topology that has one path for every node on the network. This tree originates from the root bridge. Redundant links that are not part of the shortest path tree are blocked.
It is because certain paths are blocked that a loop free topology is possible. Data frames received on blocked links are dropped.
The Spanning-Tree Protocol requires network devices to exchange messages to detect bridging loops. Links that will cause a loop are put into a blocking state.
Switches send messages called the bridge protocol data units (BPDUs) to allow the formation of a loop free logical topology. BPDUs continue to be received on blocked ports. This ensures that if an active path or device fails, a new spanning-tree can be calculated.
BPDUs contain information that allow switches to perform specific actions:
- Select a single switch that
will act as the root of the spanning-tree.
- Calculate the shortest path
from itself to the root switch.
- Designate one of the
switches as the closest one to the root, for each LAN segment. This switch
is called the designated switch. The designated switch handles all
communication from that LAN segment towards the root bridge.
- Choose one of its ports as
its root port, for each non-root switch. This is the interface that gives
the best path to the root switch.
- Select ports that are part
of the spanning-tree. These ports are called designated ports.
Non-designated ports are blocked.
The next page will describe the features of a spanning-tree network.
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