Allied Telesis AT-GS950/24, User Manual

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Appendix A: MSTP Overview
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Overview
In the AT-GS950/24, STP and RSTP are referred to as single-instance
spanning trees that search for physical loops across all VLANs in a bridged
network. When loops are detected, the active protocol stops the loops by
placing one or more bridge ports in a blocking state. See “Overview” on
page 114 for more information.
As explained in “Spanning Tree and VLANs” on page 120, STP and RSTP can
result in VLAN fragmentation where VLANs that span multiple bridges are
connected together with untagged ports. The untagged ports creating the links
can represent a physical loop in the network, which are blocked by spanning
tree. This can result in a loss of communication between different parts of the
same VLAN.
One way to resolve this, other than by not activating spanning tree on your
network, is to link the switches using tagged ports, which can handle traffic
from multiple VLANs simultaneously. The drawback to this approach is that
the link formed by the tagged ports can create a bottleneck to your Ethernet
traffic, resulting in reduced network performance.
Another approach is to use the Multiple Spanning Tree Protocol (MSTP)
feature. This spanning tree shares many of the same characteristics as RSTP
in that it features rapid convergence and has many of the same parameters.
But the main difference is that while RSTP, just like STP, supports only a
single-instance spanning tree, MSTP supports multiple spanning trees within
a network.
Note
MSTP and RSTP cannot be enabled at the same time. If RSTP is
enabled and you attempt to simultaneously enable MSTP, and error
message will be displayed saying, “ERROR: Please disable RSTP
before enabling MSTP.” Once RSTP is disabled, you may then enable
MSTP.
The following sections describe some of the terms and concepts related to
MSTP. If you are not familiar with spanning tree or RSTP, you should first
review the “Overview” on page 114.