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Chapter 4 Avaya P460 Layer 2 Features
Spanning Tree Configuration
Spanning Tree Overview
Without Spanning Tree a Network might experience packet storms when there are
multiple bridges and paths through the network. In addition, loops might be
formed in the network. When there are loops in the network Bridges see more than
one path to the same device. Packet storms and loops can cause a network to slow to
a crawl, and eventually bring the network down.
The spanning tree algorithm creates a single path through the network. The
algorithm ensures that if more than one path exists between two parts of the
network, only one of these paths is used, while the other is blocked.
The Spanning Tree Algorithm:
• Produces a logical tree topology out of any arrangement of bridges. The result is
a single path between any two end stations on an extended network.
• Provides a high degree of fault tolerance. It allows the network to automatically
reconfigure the spanning tree topology if there is a bridge or data-path failure.
The Spanning Tree Algorithm requires five values to derive the spanning tree
topology. These are:
1 A multicast address specifying all bridges on the extended network. The
software automatically determines the media-dependent address.
2 A network-unique identifier for each bridge on the extended network.
3 A unique identifier for each bridge/LAN interface (a port).
4 The relative priority of each port.
5 The cost of each port.
After these values are assigned, bridges multicast and process the formatted frames,
called Bridge Protocol Data Units, or BPDUs, to derive a single, loop-free topology
throughout the extended network. The bridges exchange BPDU frames quickly,
minimizing the time that service is unavailable between hosts.
Spanning Tree per Port
The STA can take up to 30 seconds to execute which might cause problems on ports
carrying time-sensitive traffic. You can therefore enable/disable Spanning Tree on a
per-port basis to minimize this effect.
26 Avaya P460 Configuration Guide