RuggedCom RSG2488, Руководство пользователя

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Chapter 5
Setup and Configuration
RUGGEDCOM RSG2488
User Guide
132 Benefits of MSTP
Role Description
The Master Port, which is unique in an MSTP region, is the CIST
Root Port of the CIST Regional Root, and provides the minimum
cost path to the CIST Root for all MSTIs.
Boundary Ports A Boundary Port is a port on a bridge in an MSTP region that
connects to either: a bridge belonging to a different MSTP region,
or a bridge supporting only RSTP or legacy STP. A Boundary Port
blocks or forwards all VLANs from all MSTIs and the CIST alike.
A Boundary Port may be:
• The CIST Root Port of the CIST Regional Root (and therefore
also the MSTI Master Port).
• A CIST Designated Port, CIST Alternate/Backup Port, or
Disabled. At the MSTP region boundary, the MSTI Port Role is the
same as the CIST Port Role.
A Boundary Port connected to an STP bridge will send only STP
BPDUs. One connected to an RSTP bridge need not refrain from
sending MSTP BPDUs. This is made possible by the fact that the
MSTP carries the CIST Regional Root Identifier in the field that
RSTP parses as the Designated Bridge Identifier.
Section 5.2.3.3
Benefits of MSTP
Despite the fact that MSTP is configured by default to arrive automatically at a spanning tree solution for each
configured MSTI, advantages may be gained from influencing the topology of MSTIs in an MST region. The fact
that the Bridge Priority and each port cost are configurable per MST makes it possible to control the topology of
each MSTI within a region.
Load Balancing
MSTP can be used to balance data traffic load among sets of VLANs, enabling more complete utilization of a
multiply interconnected bridged network.
A bridged network controlled by a single spanning tree will block redundant links by design, in order to avoid
harmful loops. Using MSTP, however, any given link may have a different blocking state for MSTI, as maintained
by MSTP. Any given link, therefore, might be in blocking state for some VLANs, and in forwarding state for other
VLANs, depending on the mapping of VLANs to MSTIs.
It is possible to control the spanning tree solution for each MSTI, especially the set of active links for each tree,
by manipulating, per MSTI, the bridge priority and the port costs of links in the network. If traffic is allocated
judiciously to multiple VLANs, redundant interconnections in a bridged network which, using a single spanning
tree, would have gone unused, can now be made to carry traffic.
Isolation of Spanning Tree Reconfiguration.
A link failure in an MSTP region that does not affect the roles of Boundary ports will not cause the CST to be
reconfigured, nor will the change affect other MSTP regions. This is due to the fact that MSTP information does
not propagate past a region boundary.
MSTP versus PVST
An advantage of MSTP over the Cisco Systems Inc. proprietary PVST protocol is the ability to map multiple
VLANs onto a single MSTI. Since each spanning tree requires processing and memory, the expense of keeping
track of an increasing number of VLANs increases much more rapidly for PVST than for MSTP.