Spanning Tree
Multiple Spanning Tree Overview
252 Cisco 350, 350X and 550X Series Managed Switches, Firmware Release 2.4, ver 0.4
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Multiple Spanning Tree Overview
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:
STEP 1 Set the STP Operation Mode to MSTP as described in the STP Status and Global Settings
page.
STEP 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 VLANs to a MSTP
Instance section.
STEP 3 Decide which MSTP instance be active in what VLAN, and associate these MSTP instances to
VLAN(s) accordingly.
STEP 4 Configure the MSTP attributes by:
• MSTP Properties
• MSTP Instance Settings
• VLANs to a MSTP Instance
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.
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