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 there is a DHCP assigned IP address on a switch with only one active port and Spanning Tree is enabled, the port is blocked (initial state of STP convergence). The VLAN goes down and the DHCP assigned IP address is removed from the VLAN. After STP converges, the port becomes forwarding and the DHCP IP address is restored.