Virtual Private LAN Service178FD 100/320Gbps NT and FX NT IHub Services Guide3HH-11985-AAAA-TQZZA Issue: 135.4.7.4 MSTP General PrinciplesMSTP represents modification of RSTP which allows the grouping of different VLANsinto multiple MSTIs. To enable different devices to participate in MSTIs, they mustbe consistently configured. A collection of interconnected devices that have thesame MST configuration (region-name, revision and VLAN-to-instance assignment)comprises an MST region.There is no limit to the number of regions in the network, but every region can supporta maximum of 1615 MST instances, with different VLAN ranges. VLANs which arenot included explicitly in an MSTP instance fall into the default MSTP instance 0.Instance 0 is a special instance for a region, known as the Internal Spanning Tree(IST) instance. All other instances are numbered 1 to 4093. IST is the onlyspanning-tree instance that sends and receives BPDUs (typically BPDUs areuntagged). All other spanning-tree instance information is included in MSTP records(M-records), which are encapsulated within MSTP BPDUs. This means that singleBPDU carries information for multiple MSTI which reduces overhead of the protocol.Any given MSTI is local to an MSTP region and completely independent from anMSTI in other MST regions. Two redundantly connected MST regions will use only asingle path for all traffic flows (no load balancing between MST regions or betweenMST and SST region).Traditional L2 switches running MSTP protocol assign all VLANs to the IST instanceper default. The operator may then “re-assign” individual VLANs to a given MSTI byconfiguring per VLAN assignment. This means that an ISAM can be considered asthe part of the same MST region only if the VLAN assignment to IST and MSTIs isidentical to the one of L2 switches in access network.5.4.7.5 MSTP implementationThe ISAM uses m-VPLS to group different SAPs under a single STP instance. TheVLAN range covering SAPs to be managed by a given m-VPLS is declared under aspecific m-VPLS SAP definition. MSTP mode-of-operation is only supported in anm-VPLS.When running MSTP, by default, all VLANs are mapped to the CIST. On the m-VPLSlevel VLANs can be assigned to specific MSTIs.5.4.7.6 Pseudowire Control WordThe control word command enables the use of the control word individually on eachmesh-sdp or spoke-sdp. By default, the control word is disabled. When the controlword is enabled, all VPLS packets, including the BPDU frames are encapsulated withthe control word.
