Spanning Tree
Multiple Spanning Tree
Cisco Small Business 300 Series Managed Switch Administration Guide 226
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Multiple Spanning Tree
Multiple Spanning Tree Protocol (MSTP) is used to separate the STP port state
between various domains (on different VLANs). For example, while port A is
blocked in one STP instance due to a loop on VLAN A, the same port can be
placed in the Forwarding State in another STP instance. The MSTP Properties
page enables you to define the global MSTP settings.
To configure MSTP:
1. Set the STP Operation Mode to MSTP as described in the Configuring STP
Status and Global Settings page.
2. Define MSTP instances. Each MSTP instance calculates and builds a loop free
topology to bridge packets from the VLANs that map to the instance. Refer to
the Mapping VLANs to a MSTP Instance section.
3. Decide which MSTP instance be active in what VLAN, and associate these
MSTP instances to VLAN(s) accordingly.
4. Configure the MSTP attributes by:
•Defining MSTP Properties
•Defining MSTP Instance Settings
•Mapping VLANs to a MSTP Instance
Defining MSTP Properties
The global MSTP configures a separate Spanning Tree for each VLAN group and
blocks all but one of the possible alternate paths within each spanning tree
instance. MSTP enables formation of MST regions that can run multiple MST
instances (MSTI). Multiple regions and other STP bridges are interconnected using
one single common spanning tree (CST).
MSTP is fully compatible with RSTP bridges, in that an MSTP BPDU can be
interpreted by an RSTP bridge as an RSTP BPDU. This not only enables
compatibility with RSTP bridges without configuration changes, but also causes
any RSTP bridges outside of an MSTP region to see the region as a single RSTP
bridge, regardless of the number of MSTP bridges inside the region itself.
For two or more switches to be in the same MST region, they must have the same
VLANs to MST instance mapping, the same configuration revision number, and the
same region name.