25. Spanning Tree
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RuggedBackbone™ RX5000
• A CIST Designated Port, CIST Alternate / Backup Port, or Disabled. At the MST 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.
25.2.3. 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 MSTI (see
Section 25.4.4, “Port MSTI Parameters”
) makes it possible to control the topology of each MSTI within
a region.
Load Balancing
MST 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
each spanning tree instance (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 MST region that does not affect the roles of Boundary ports will not cause the CST
to be reconfigured, nor will the change affect other MST 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.
Compatibility with STP and RSTP
No special configuration is required for the bridges of an MST region to connect fully and simply
to non-MST bridges on the same bridged network. Careful planning and configuration is, however,
recommended in order to arrive at an optimal network.
25.2.4. Implementing MSTP on a Bridged Network
It is recommended that the configuration of MSTP on a network proceed in the sequence outlined below.
Naturally, it is also recommended that network analysis and planning inform the steps of configuring
the VLAN and MSTP parameters in particular.
Begin with a set of MSTP-capable Ethernet bridges, and MSTP disabled. For each bridge in the network:
