Allied Telesis AT-S62 - Manual (Page 353)

AT-S62 User’s Guide
Section IV: Spanning Tree Protocols 353
MSTP Overview
As explained in the previous chapter, STP and RSTP are 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 338, activating STP or
RSTP can result in VLAN fragmentation when VLANs that span multiple
bridges are interconnected with untagged ports. The untagged ports
creating the links can represent a physical loop in the network, which
will be 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 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 the terms and concepts of MSTP. If you
are not familiar with spanning tree or RSTP, you should first review the
section STP and RSTP Overview on page 330.
Note
Do not activate MSTP on an AT-8524M 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.
Note
The AT-S62 implementation of MSTP complies with the IEEE 802.1s
standard and is compatible with versions from other vendors that
conform to the standard.