Configuring the Spanning Tree Protocol
STP Flavors
Cisco Small Business 300 Series Managed Switch Administration Guide 128
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The switch supports the following Spanning Tree Protocol versions:
• Classic STP provides a single path between any two end stations, avoiding
and eliminating loops.
• Rapid STP (RSTP) detects network topologies to provide faster
convergence of the spanning tree. This 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 each bridge or switch in the network needs
to decide, if it should actively forward traffic or not on each of its ports.
• Multiple STP (MSTP) 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
instance is associated with the Layer 2 domain on which it performs loop
detection and mitigation. This enables a port to be stopped in one instance,
such as traffic from VLAN A that is causing a loop, while traffic can remain
active in another domain where no loop was seen, such as on VLAN B.
MSTP provides full connectivity for packets allocated to any VLAN. MSTP is
based on RSTP. In addition, MSTP transmits packets assigned to various
VLANs through different multiple spanning tree (MST) regions. MST regions
act as a single bridge.
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