Spanning Tree Algorithm
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Instruction Manual - NXA-ENET8-POE+
MSTP
- When using STP or RSTP, it may be difficult to maintain a stable path between all VLAN members. Frequent changes in the
tree structure can easily isolate some of the group members. MSTP (which is based on RSTP for fast convergence) is designed to
support independent spanning trees based on VLAN groups. Using multiple spanning trees can provide multiple forwarding paths
and enable load balancing. One or more VLANs can be grouped into a Multiple Spanning Tree Instance (MSTI). MSTP builds a
separate Multiple Spanning Tree (MST) for each instance to maintain connectivity among each of the assigned VLAN groups. MSTP
then builds a Internal Spanning Tree (IST) for the Region containing all commonly configured MSTP bridges.
An MST Region consists of a group of interconnected bridges that have the same MST Configuration Identifiers (including the
Region Name, Revision Level and Configuration Digest - see the
Configuring Multiple Spanning Trees
Region may contain multiple MSTP Instances. An Internal Spanning Tree (IST) is used to connect all the MSTP switches within an
MST region. A Common Spanning Tree (CST) interconnects all adjacent MST Regions, and acts as a virtual bridge node for
communications with STP or RSTP nodes in the global network.
MSTP connects all bridges and LAN segments with a single Common and Internal Spanning Tree (CIST). The CIST is formed as a
result of the running spanning tree algorithm between switches that support the STP, RSTP, MSTP protocols.
Once you specify the VLANs to include in a Multiple Spanning Tree Instance (MSTI), the protocol will automatically build an MSTI
tree to maintain connectivity among each of the VLANs. MSTP maintains contact with the global network because each instance is
treated as an RSTP node in the Common Spanning Tree (CST).
FIG. 110
MSTP Region, Internal Spanning Tree, Multiple Spanning Tree
FIG. 111
Spanning Tree - Common Internal, Common, Internal
IST
(for this region)
Region R
MST 1
MST 2
CIST
IST
Region 1
Region 4
Region 3
Region 2
CST
Region 4
Region 3
Region 2
Region 1