Multiple Spanning Tree - Alcatel-Lucent 7750 SR OS Service Manual

VPLS Features
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The 7750 SR supports two BDPU encapsulation formats, and can dynamically switch between
these (again on a per-SAP basis):
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Multiple Spanning Tree

The Multiple Spanning Tree Protocol (MSTP) extends the concept of the IEEE 802.1w Rapid
Spanning Tree Protocol (RSTP) by allowing grouping and associating VLANs to Multiple
Spanning Tree Instances (MSTI). Each MSTI can have its own topology, which provides
architecture enabling load balancing by providing multiple forwarding paths. At the same time, the
number of STP instances running in the network is significantly reduced as compared to Per
VLAN STP (PVST) mode of operation. Network fault tolerance is also improved because a failure
in one instance (forwarding path) does not affect other instances.
The 7x50 implementation of Management VPLS (mVPLS) is used to group different VPLS
instances under single RSTP instance. Introducing MSTP into the mVPLS allows interoperating
with traditional Layer 2 switches in access network and provides an effective solution for dual
homing of many business Layer 2 VPNs into a provider network.
Redundancy Access to VPLS
The GigE MAN portion of the network is implemented with traditional switches. Using MSTP
running on individual switches facilitates redundancy in this part of the network. In order to
provide dual homing of all VPLS services accessing from this part of the network, the VPLS PEs
must participate in MSTP.
This can be achieved by configuring mVPLS on VPLS-PEs (only PEs directly connected to GigE
MAN network) and then assign different managed-vlan ranges to different MSTP instances.
Typically, the mVPLS would have SAPs with null encapsulations (to receive, send, and transmit
MSTP BPDUs) and a mesh SDP to interconnect a pair of VPLS PEs.
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The RSTP mode implements the improved convergence over shared media feature, for
example, RSTP will transition from discarding to forwarding in 4 seconds when operating
over shared media. The comp-dot1w mode does not implement this 802.1D-2004
improvement and transitions conform to 802.1w in 30 seconds (both modes implement
fast convergence over point-to-point links).
In the RSTP mode, the transmitted BPDUs contain the port's designated priority vector
(DPV) (conforms to 802.1D-2004). Older implementations may be confused by the DPV
in a BPDU and may fail to recognize an agreement BPDU correctly. This would result in a
slow transition to a forwarding state (30 seconds). For this reason, in the comp-dot1w
mode, these BPDUs contain the port's port priority vector (conforms to 802.1w).
IEEE 802.1D STP
Cisco PVST
7750 SR OS Services Guide