Preparing for MSTP configuration

  • Ensure that the VLAN configuration in your network supports all of 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 MST 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 MSTI 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 MSTI.
  • Determine the designated bridge and designated port for each LAN segment.
  • Determine which VLANs to assign to each MST 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.
  • Set the admin-edge port type to admin-edge for edge ports connected to end nodes.
  • Set the admin-edge port type to admin-network 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 is limited to four. However, when the switch is configured as a standalone, a maximum of 16 MSTIs is supported. For the purposes of this guide, all examples assume that the switch is standalone; therefore, a maximum of 16 instances are displayed.