Spanning Tree Protocol Overview - Dell PowerConnect 5524P User Manual

Spanning Tree Protocol Overview

Spanning Tree Protocol (STP) provides tree topography for any bridge
arrangement. STP eliminates loops by providing a unique path between end
stations on a network.
Loops occur when alternate routes exist between hosts. Loops, in an extended
network, can cause bridges to forward traffic indefinitely, resulting in packets
not arriving at their destination, increased traffic, and reduced network
efficiency.
The device supports the following Spanning Tree versions:
• Classic STP — Provides a single path between end stations, avoiding and
eliminating loops. For more information on configuring Classic STP, see
"Global Settings" on page 431.
Rapid STP (RSTP) — Provides faster convergence of the spanning tree
•
than Classic STP. RSTP is most effective when the network topology is
naturally tree-structured, and therefore faster convergence might be
possible. RSTP is enabled by default.
Although Classic STP is guaranteed to prevent Layer 2 forwarding loops, in
a general network topology, there might be an unacceptable delay before
convergence. This means that before convergence, each bridge or switch in
the network must decide if it should actively forward traffic or not, on each
of its ports.
For more information on configuring Rapid STP, see "Rapid Spanning
Tree" on page 444.
• Multiple STP (MSTP) — MSTP is based on RSTP. It detects Layer 2
loops, and attempts to mitigate them by preventing the involved port from
transmitting traffic.
Since loops exist on a per-Layer 2-domain basis, a situation can occur
where there is a loop in VLAN A and no loop in VLAN B. If both VLANs
are on Port X, and STP wants to mitigate the loop, it stops traffic on the
entire port, including VLAN B traffic, where there is no need to stop
traffic.
Multiple Spanning Tree Protocol (MSTP) solves this problem by enabling
several STP instances, so that it is possible to detect and mitigate loops
separately in each instance. By associating instances to VLANs, each
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