Advanced Traffic Management Guide K/KA/KB.15.15

• Within an MSTI, there is one physical communication path between any two nodes, regardless
of how many VLANs belong to the MSTI. Within an IST instance, there is also one spanning
tree across all VLANs belonging to the IST instance.
• An MSTI comprises a unique set of VLANs and forms a single spanning tree instance within
the region to which it belongs.
• A dynamic VLAN learned by GVRP will always be placed in the IST instance and cannot be
moved to any configured MST instance.
Starting in software release 13.X.X, dynamically learned GVRP VLANs can be mapped to
MSTIs and support MSTP load balancing.
• In software release 13.X.X and later, you can preconfigure static and dynamic VLAN ID-to-MSTI
mappings before the VLAN is created on the switch. Later, when the static VLAN ID is
configured or a dynamic GVRP VLAN is learned, the VLAN is automatically associated with
the preconfigured MSTI.
• Communication between MST regions uses a single spanning tree.
• If a port on a switch configured for MSTP receives a legacy (STP/802.1D or RSTP/802.1w)
BPDU, it automatically operates as a legacy port. In this case, the MSTP switch interoperates
with the connected STP or RSTP switch as a separate MST region.
• Within an MST region, there is one logical forwarding topology per instance, and each
instance comprises a unique set of VLANs. Where multiple paths exist between a pair of nodes
using VLANs belonging to the same instance, all but one of those paths will be blocked for
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).
• MSTP interprets a switch mesh as a single link.
Operating notes for the VLAN configuration enhancement
• Configuring MSTP on the switch automatically configures the Internal Spanning Tree (IST)
instance and places all statically and dynamically configured VLANs on the switch into the
IST instance. The spanning tree instance vlan command creates a new MST instance and
moves the VLANs you specify from the IST to the MSTI.
You must map a least one VLAN ID to an MSTI when you create it. You cannot map a VLAN
ID to more than one instance. You can create up to 16 MSTIs in a region.
• The no form of the spanning tree instance vlan command removes one or more VLANs from
the specified MSTI. If no VLANs are specified, the no form of the command deletes the specified
MSTI.
When you remove a VLAN from an MSTI, the VLAN returns to the IST instance, where it can
remain or be reassigned to another MSTI configured in the region.
• If you enter the spanning tree instance vlan command before a static or dynamic VLAN is
configured on the switch to preconfigure VLAN ID-to-MSTI mappings, no error message is
displayed. Later, each newly configured VLAN that has already been associated with an MSTI
is automatically assigned to the MSTI.
This new default behavior differs from automatically including configured (static and dynamic)
VLANs in the IST instance and requiring you to manually assign individual static VLANs to an
MSTI.
• The valid VLAN IDs that you can map to a specified MSTI are from 1 to 4094. The VLAN
ID-to-MSTI mapping does not require a VLAN to be already configured on the switch. The
126 Multiple instance spanning tree operation