Spanning-Tree Operation
802.1s Multiple Spanning Tree Protocol (MSTP)
MST Region: An MST region comprises the VLANs configured on physically
connected MSTP switches. All switches in a given region must be configured
with the same VLANs and Multiple Spanning Tree Instances (MSTIs).
Internal Spanning Tree (IST): The IST administers the topology within a
given MST region. 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). (Refer to “Multiple Spanning Tree Instance”, below.)
Types of 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 includes all VLANs in the network. (An STP or RSTP network operates as
a single-instance network.) A region can include two types of STP instances:
■ Internal Spanning-Tree Instance (IST Instance): This is the default
spanning tree instance in any MST region. It provides the root switch for
the region and comprises all VLANs configured on the switches in the
region that are not specifically assigned to Multiple Spanning Tree
Instances (MSTIs, described below). All VLANs in the IST instance of a
region are part of the same, single spanning tree topology, which allows
only one forwarding path between any two nodes belonging to any of the
VLANs included in the IST instance. All switches in the region must belong
to the set of VLANs that comprise the IST instance. Note that the switch
automatically places dynamic VLANs (resulting from GVRP operation) in
the IST instance. Dynamic VLANs cannot exist in an MSTI (described
below).
■ MSTI (Multiple Spanning Tree Instance): This type of configurable
spanning tree instance comprises all static VLANs you specifically assign
to it, and must include at least one VLAN. The VLAN(s) you assign to an
MSTI must initially exist in the IST instance of the same MST region. When
you assign a static VLAN to an MSTI, the switch removes the VLAN from
the IST instance. (Thus, you can assign a VLAN to only one MSTI in a given
region.) All VLANs in an MSTI operate as part of the same single spanning
tree topology. (The switch does not allow dynamic VLANs in an MSTI.)
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