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Multiple Spanning Tree Protocol
IEEE 802.1s Multiple Spanning Tree extends the IEEE 802.1w Rapid Spanning Tree Protocol through multiple
Spanning Tree Groups. MSTP maintains up to 32 spanning-tree instances that correspond to STP Groups 1-32.
In Multiple Spanning Tree Protocol (MSTP), several VLANs can be mapped to each Spanning-Tree instance. Each
Spanning-Tree instance is independent of other instances. MSTP allows frames assigned to different VLANs to
follow separate paths, each path based on an independent Spanning-Tree instance. This approach provides
multiple forwarding paths for data traffic, enabling load balancing, and reducing the number of Spanning-Tree
instances required to support a large number of VLANs.
MSTP region
A group of interconnected bridges that share the same attributes is called an MSTP region. Each bridge within the
region must share the following attributes:
•
Alphanumeric name
•
Revision level
•
VLAN-to-STG mapping scheme
MSTP provides rapid reconfiguration, scalability, and control due to the support of regions, and multiple Spanning-
Tree instances support within each region.
Common Internal Spanning Tree
The Common Internal Spanning Tree (CIST) provides a common form of Spanning Tree Protocol, with one
Spanning Tree instance that can be used throughout the MSTP region. CIST allows the switch to interoperate with
legacy equipment, including devices that run IEEE 802.1D (STP).
CIST allows the MSTP region to act as a virtual bridge to other bridges outside of the region, and provides a single
Spanning-Tree instance to interact with them.
CIST is the default spanning tree group. When VLANs are removed from STG 1-32, the VLANs automatically
become members of the CIST.
CIST port configuration includes Hello time, Edge port status (enable/disable), and Link Type. These parameters do
not affect Spanning Tree Groups 1-32. They apply only when the CIST is used.
MSTP configuration guidelines
This section provides important information about configuring Multiple Spanning Tree Groups:
•
When you turn on MSTP, the switch automatically moves VLAN 1 to the Common Internal Spanning Tree
(CIST).
•
Region Name and revision level must be configured. Each bridge in the region must have the same name and
revision level.
•
The VLAN and STP Group mapping must be the same across all bridges in the region.
•
You can move any VLAN to the CIST.
•
You can move VLAN 1 into any Spanning Tree Group.
MSTP configuration example
This section provides steps to configure Multiple Spanning Tree Protocol on the switch, using the Command Line
Interface (CLI) or the Browser-based Interface (BBI).
Configuring Multiple Spanning Tree Protocol (CLI example)
1.
Configure port and VLAN membership on the switch, as described in the "Configuring ports and VLANs (CLI
example)" section in the "VLANs" chapter of this guide.
2.
Set the mode to Multiple Spanning Tree, and configure MSTP region parameters.
>> /cfg/l2/ mrst
>> Multiple Spanning Tree# mode mstp
>> Multiple Spanning Tree# on
>> Multiple Spanning Tree# name xxxxxx (Define the Region name)
>> Multiple Spanning Tree: rev xx
(Select Multiple Spanning Tree menu)
(Set mode to
Multiple Spanning Trees)
(Turn Multiple Spanning Trees on)
(Define the Region revision level)
RSTP and MSTP 59
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