KB.15.15

3 Multiple instance spanning tree operation

NOTE: All commands previously in the Summary of commands table are indexed under the entry

Command syntax.

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 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 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 inter-regional links•

• Any IST (Internal Spanning Tree) or MST 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.

• 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.

Configuring MSTP at a glance

The general steps for configuring MSTP via the CLI are:

Planning an MSTP application 77