VPLS Features
Page 268 7210 SAS M Services Guide
MSTP General Principles
MSTP represents modification of RSTP which allows the grouping of different VLANs into
multiple MSTIs. To enable different devices to participate in MSTIs, they must be consistently
configured. A collection of interconnected devices that have the same 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 support a maximum
of 16 MSTIs. Instance 0 is a special instance for a region, known as the Internal Spanning Tree
(IST) instance. All other instances are numbered from 1 to 4094. IST is the only spanning-tree
instance that sends and receives BPDUs (typically BPDUs are untagged). All other spanning-tree
instance information is included in MSTP records (M-records), which are encapsulated within
MSTP BPDUs. This means that single BPDU carries information for multiple MSTI which
reduces overhead of the protocol.
Any given MSTI is local to an MSTP region and completely independent from an MSTI in other
MST regions. Two redundantly connected MST regions will use only a single path for all traffic
flows (no load balancing between MST regions or between MST and SST region).
Traditional Layer 2 switches running MSTP protocol assign all VLANs to the IST instance per
default. The operator may then “re-assign” individual VLANs to a given MSTI by configuring per
VLAN assignment. This means that a SR-Series PE can be considered as the part of the same MST
region only if the VLAN assignment to IST and MSTIs is identical to the one of Layer 2 switches
in access network.
MSTP in the 7210 SAS Platform
The 7210 SAS platform uses a concept of mVPLS to group different SAPs under a single STP
instance. The VLAN range covering SAPs to be managed by a given mVPLS is declared under a
specific mVPLS SAP definition. MSTP mode-of-operation is only supported in an mVPLS.
When running MSTP, by default, all VLANs are mapped to the CIST. On the VPLS level VLANs
can be assigned to specific MSTIs. When running RSTP, the operator must explicitly indicate, per
SAP, which VLANs are managed by that SAP.
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