Allied Telesis AT-GS950/8 - Manual (Page 368)

Appendix A: MSTP Overview
368
Overview
In the AT-GS950/8, 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 Chapter 4, “STP and
RSTP” on page 71 for more information.
As explained in “Spanning Tree and VLANs” on page 78, 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.
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 Chapter 4, “STP and RSTP” on page 71.
Note
Do not activate MSTP on the AT-GS950/8 switch without first
familiarizing yourself with the following concepts and guidelines. Like
STP and RSTP, you must activate this MSTP protocol on a switch and
then configure the protocol parameters.