Siemens RUGGEDCOM ROX II User Manual page 686

Chapter 14
Network Redundancy
Role
Boundary Ports
Section 14.3.3.3
Benefits of MSTP
MSTP is configured by default to arrive automatically at a spanning tree solution for each configured MSTI.
However, advantages may be gained from influencing the topology of MSTIs in an MST region by way of the
Bridge Priority and the cost of each port.
Load Balancing
MSTP can be used to balance the data traffic load among sets of VLANs, enabling more complete utilization of a
bridged network that has multiple redundant interconnections between bridges.
A bridged network controlled by a single spanning tree will block redundant links by design to avoid harmful
loops. However, when using MSTP, 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 would have gone unused
when using a single spanning tree, 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 Per-VLAN Spanning Tree (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.
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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.
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.
RUGGEDCOM ROX II
CLI User Guide
Benefits of MSTP