Terminology - HP procurve 8100fl series Management And Configuration Manual

Spanning-Tree Operation
802.1s Multiple Spanning Tree Protocol (MSTP)
9-6

Terminology

Bridge: See "MSTP Bridge".
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. (Refer to figure 9-2 on page 9-5.)
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). (Refer to "Multiple Spanning Tree Instance", below.)
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
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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.