Advanced Traffic Management Guide K/KA/KB.15.15

that instance. However, if there are different paths in different instances, all such paths are
available for traffic. Separate forwarding paths exist through separate spanning tree instances.
• A port can have different states (forwarding or blocking) for different instances (which represent
different forwarding paths.)
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 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.
• 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 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 edge-port setting for these
ports. Leave the edge-port setting disabled for ports connected to another switch, a bridge,
or a hub.
Terminology
Common and Internal Spanning Tree (CIST): Comprises all LANs, STP, and RSTP bridges and MSTP
regions in a network. The CIST automatically determines the MST regions in a network and defines
the root bridge (switch) and designated port for each region. The CIST includes the Common
Spanning Tree (CST), the Internal Spanning Tree (IST) within each region, and any multiple
spanning-tree instances (MSTIs) in a region.
Common Spanning Tree (CST): Administers the connectivity among the MST regions, STP LANs,
and RSTP LANs in a bridged network. CST refers to the single forwarding path the switch calculates
for STP (802.1D) and RSTP (802.1w) topologies, and for inter-regional paths in MSTP (802.1s)
topologies. Note that all three versions of spanning tree can interoperate in the same network.
Also, the MSTP switch interprets a device running 802.1D STP or 802.1w RSTP as a separate
region.
Internal Spanning Tree (IST): When you configure a switch for MSTP operation, the switch
automatically includes all of the static VLANs configured on the switch in a single, active spanning
tree topology (instance) within the IST. This is termed the “IST instance”. Any VLANs you subsequently
configure on the switch are added to this IST instance. To create separate forwarding paths within
a region, group specific VLANs into different Multiple Spanning Tree Instances (MSTIs).
Multiple Spanning Tree Instances: A multiple spanning tree network comprises separate spanning-tree
instances existing in an MST region. (There can be multiple regions in a network.) Each instance
defines a single forwarding topology for an exclusive set of VLANs. By contrast, an STP or RSTP
network has only one spanning tree instance for the entire network, and
Terminology 451