Siemens RUGGEDCOM ROX II User Manual page 684

Chapter 14
Network Redundancy
By design, MSTP processing time is proportional to the number of active STP instances. This means MSTP will likely
be significantly slower than RSTP. Therefore, for mission critical applications, RSTP should be considered a better
network redundancy solution than MSTP.
CONTENTS
• Section 14.3.3.1, "MSTP Regions and Interoperability"
• Section 14.3.3.2, "MSTP Bridge and Port Roles"
• Section 14.3.3.3, "Benefits of MSTP"
• Section 14.3.3.4, "Implementing MSTP on a Bridged Network"
Section 14.3.3.1
MSTP Regions and Interoperability
In addition to supporting multiple spanning trees in a network of MSTP-capable bridges, MSTP is capable of inter-
operating with bridges that support only RSTP or legacy STP, without requiring any special configuration.
An MST region may be defined as the set of interconnected bridges whose MST Region Identification is identical.
The interface between MSTP bridges and non-MSTP bridges, or between MSTP bridges with different MST Region
Identification information, becomes part of an MST Region boundary.
Bridges outside an MST region will see the entire region as though it were a single (R)STP bridge, with the internal
detail of the MST region being hidden from the rest of the bridged network. In support of this, MSTP maintains
separate hop counters for spanning tree information exchanged at the MST region boundary versus information
propagated inside the region. For information received at the MST region boundary, the (R)STP Message Age is
incremented only once. Inside the region, a separate Remaining Hop Count is maintained, one for each spanning
tree instance. The external Message Age parameter is referred to the (R)STP Maximum Age Time, whereas the
internal Remaining Hop Counts are compared to an MST region-wide Maximum Hops parameter.
MSTI
An MSTI (Multiple Spanning Tree Instance) is one of sixteen independent spanning tree instances that may be
defined in an MST region (not including the IST). An MSTI is created by mapping a set of VLANs to a given MSTI ID.
The same mapping must be configured on all bridges that are intended to be part of the MSTI. Moreover, all VLAN-
to-MSTI mappings must be identical for all bridges in an MST region.
RUGGEDCOM ROX II supports 16 MSTIs in addition to the IST.
Each MSTI has a topology that is independent of others. Data traffic originating from the same source and bound
to the same destination, but on different VLANs on different MSTIs, may therefore travel a different path across
the network.
IST
An MST region always defines an IST (Internal Spanning Tree). The IST spans the entire MST region, and carries all
data traffic that is not specifically allocated (by VLAN) to a specific MSTI. The IST is always computed and is defined
to be MSTI zero.
The IST is also the extension inside the MST region of the CIST
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RUGGEDCOM ROX II
CLI User Guide
MSTP Regions and Interoperability