Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

incompatibility between devices running the older 802.1D STP and your switch running MSTP or RSTP. Please see the “Note on Path Cost” on page 5-19 for more information on adjusting to this incompatibility.

Tips for Planning an MSTP Application

Ensure that the VLAN configuration in your network supports all of the forwarding paths necessary for the desired connectivity. All ports con­ necting one switch to another within a region and one switch to another between regions should be configured as members of all VLANs config­ ured in the region.

All ports or trunks connecting one switch to another within a region should be configured 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.)

There is one logical spanning-tree path through the following:

Any inter-regional links

Any IST 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 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 seg­ ment.

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. (Refer to “MSTP Operation with 802.1Q VLANs” on page 5-51.)

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

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