Chapter 5Setup and ConfigurationRUGGEDCOM ROX IIUser Guide702 MSTP Regions and InteroperabilityBy design, MSTP processing time is proportional to the number of active STP instances. This means MSTP willlikely be significantly slower than RSTP. Therefore, for mission critical applications, RSTP should be considered abetter network redundancy solution than MSTP.The following sections further describe the operation of MSTP:• Section 5.34.3.1, “MSTP Regions and Interoperability”• Section 5.34.3.2, “MSTP Bridge and Port Roles”• Section 5.34.3.3, “Benefits of MSTP”• Section 5.34.3.4, “Implementing MSTP on a Bridged Network”Section 5.34.3.1MSTP Regions and InteroperabilityIn addition to supporting multiple spanning trees in a network of MSTP-capable bridges, MSTP is capable ofinter-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 MSTRegion 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 theinternal detail of the MST region being hidden from the rest of the bridged network. In support of this, MSTPmaintains separate hop counters for spanning tree information exchanged at the MST region boundary versusinformation propagated inside the region. For information received at the MST region boundary, the (R)STPMessage 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 MaximumAge Time, whereas the internal Remaining Hop Counts are compared to an MST region-wide Maximum Hopsparameter.MSTIAn MSTI (Multiple Spanning Tree Instance) is one of sixteen independent spanning tree instances that may bedefined in an MST region (not including the IST). An MSTI is created by mapping a set of VLANs to a given MSTIID. The same mapping must be configured on all bridges that are intended to be part of the MSTI. Moreover, allVLAN-to-MSTI mappings must be identical for all bridges in an MST region.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 boundto the same destination, but on different VLANs on different MSTIs, may therefore travel a different path acrossthe network.ISTAn MST region always defines an IST (Internal Spanning Tree). The IST spans the entire MST region, andcarries all data traffic that is not specifically allocated (by VLAN) to a specific MSTI. The IST is always computedand is defined to be MSTI zero.The IST is also the extension inside the MST region of the CISTCSTThe CST (Common Spanning Tree) spans the entire bridged network, including MST regions and any connectedSTP or RSTP bridges. An MST region is seen by the CST as an individual bridge, with a single cost associatedwith its traversal.
