Reference Guide

312 Spanning Tree
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 .
•
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 e
xist on a per-Layer 2-domain basis, a situation can
occur
w
here there is a loop in VLAN A and no loop in VLAN B. If both
VLANs
ar
e on Port X, and STP wants to mitigate the loop, it stops traffic
on the
e
ntire port, including VLAN B traffic, where there is no need to
stop
tr
affic.
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, and
transmits packets assigned to various VLANs, through different multip
le
s
panning tree (MST) regions.
MST regions act as a single bridge.
For more information on configuring Multiple STP, see
Multiple
Spanning Tree
.