5-16
Multiple Instance 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 5-5
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
5-11.)
■ 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 hub.
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