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Mstp overview – Allied Telesis AT-S62 User Manual

Page 508

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Chapter 23: Multiple Spanning Tree Protocol

508

Section IV: Spanning Tree Protocols

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 491, 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 communications
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 484.

Note

Do not activate MSTP on an AT-8500 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.

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.