Planning an MSTP application

Before configuring MSTP, keep in mind the following tips and considerations:
  • Ensure that the VLAN configuration in your network supports all the forwarding paths necessary for the desired connectivity. All ports connecting one switch to another within a region and one switch to another between regions should be configured as members of all VLANs configured in the region.

  • Configure all ports or trunks connecting one switch to another within a region as members of all VLANs in the region. Otherwise, some VLANs could be blocked from access to the spanning tree root for an instance or for the region.

  • Plan individual regions based on VLAN groupings. That is, plan on all MSTP switches in a given region supporting the same set of VLANs. Within each region, determine the VLAN membership for each spanning tree instance. (Each instance represents a single forwarding path for all VLANs in that instance.)

  • Verify that there is one logical spanning tree path through the following:
    • Any interregional links

    • Any IST (Internal Spanning Tree) or Multiple Spanning Tree Instance within a region

    • Any legacy (802.1D or 802.1w) switch or group of switches. (Where multiple paths exist between an MST region and a legacy switch, expect the CST (Common Spanning Tree) to block all but one such path.)

  • Determine the root bridge and root port for each instance.

  • Determine the designated bridge and designated port for each LAN segment.

  • Determine which VLANs to assign to each instance and use port trunks with 802.1Q VLAN tagging where separate links for separate VLANs would result in a blocked link preventing communication between nodes on the same VLAN. (See MSTP operation with 802.1Q VLANs .)

  • Identify the edge ports connected to end nodes and enable the admin-edge-port setting for these ports. Leave the admin-edge-port setting disabled for ports connected to another switch, a bridge, or a half-duplex repeater.


When the Switch is configured in a stack, the number of configurable MSTIs has now been increased to 16, from 4. This is same as a standalone switch.

For the purposes of this guide, all examples assume that the Switch is standalone; therefore, a maximum of 16 instances are displayed.