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Chapter 11 Avaya P330 Layer 2 Features
Avaya P333R-LB User’s Guide 71
Spanning Tree Protocol
Overview
Avaya P330 devices support both common Spanning Tree protocol (802.1d) and the
enhanced Rapid Spanning Tree protocol (802.1w). The 802.1w is a faster and more
sophisticated version of the 802.1d (STP) standard. Spanning Tree makes it possible
to recover connectivity after an outage within a minute or so. RSTP, with its “rapid”
algorithm, can restore connectivity to a network where a backbone link has failed in
much less time.
In order to configure the switch to either common Spanning Tree or Rapid Spanning
Tree protocol, use the set spantree version command.
Spanning Tree Protocol
The Spanning Tree Algorithm ensures the existence of a loop-free topology in
networks that contain parallel bridges. A loop occurs when there are alternate
routes between hosts. If there is a loop in an extended network, bridges may
forward traffic indefinitely, which can result in increased traffic and degradation in
network performance.
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. This
address is media-dependent and is automatically determined by the software.
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 Spanning Tree can take up to 30 seconds to open traffic on a port. This delay can
cause problems on ports carrying time-sensitive traffic. You can therefore enable/
disable Spanning Tree in P330 on a per-port basis to minimize this effect.