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RUGGEDCOM ROX II
CLI User Guide
The following sections further describe the operation of MSTP:
•
Section 5.36.3.1, "MSTP Regions and Interoperability"
•
Section 5.36.3.2, "MSTP Bridge and Port Roles"
•
Section 5.36.3.3, "Benefits of MSTP"
•
Section 5.36.3.4, "Implementing MSTP on a Bridged Network"
Section 5.36.3.1
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
CST
The CST (Common Spanning Tree) spans the entire bridged network, including MST regions and any connected
STP or RSTP bridges. An MST region is seen by the CST as an individual bridge, with a single cost associated
with its traversal.
MSTP Regions and Interoperability
Chapter 5
Setup and Configuration
523
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