Allied Telesis AT-S60 - Manual (Page 258)

Chapter 16: Multiple Spanning Tree
Section II: Advanced Features 258
MSTP Overview
As mentioned in Chapter 15, STP and RSTP on page 228, 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 protocols stop the loops by placing one or more bridge ports in a
blocking state.
As explained in Spanning Tree and VLANs on page 237, 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. The result can be 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).
This spanning tree shares many of the same characteristics as RSTP. 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 relating
to MSTP. If you are not familiar with spanning tree or RSTP, you should
first review the section on page 257.
Note
Do not activate MSTP on an AT-8400 Series switch without first
familiarizing yourself with the following concepts and guidelines.
Unlike STP and RSTP, you cannot activate this spanning tree
protocol on a switch without first configuring the protocol
parameters.