Allied Telesis AT-S63, Features Manual

Page: 249 / 538

AT-S63 Management Software Features Guide
Section V: Spanning Tree Protocols 249
Overview
As mentioned in Chapter 22, ”Spanning Tree and Rapid Spanning Tree
Protocols” on page 235, 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 246, 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).
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 “Overview” on page 237.
Note
Do not activate MSTP on the AT-9400 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-S63 MSTP implementation complies fully with the new IEEE
802.1s standard and should be interoperable with any other
vendor’s fully compliant 802.1s implementation.