You jumped ahead to configuring MST instead of reading the Cisco documentation describing MST, Understanding Multiple Spanning Tree Protocol (802.1s). This document has multiple, colored diagrams, and a pretty good explanation of terms.
The IEEE 802.1s committee adopted a much easier and simpler approach
that introduced MST regions. Think of a region as the equivalent of
Border Gateway Protocol (BGP) Autonomous Systems, which is a group of
switches placed under a common administration.
In order to clearly understand the role of the IST instance, remember
that MST originates from the IEEE. Therefore, MST must be able to
interact with 802.1q-based networks, because 802.1q is another IEEE
standard. For 802.1q, a bridged network only implements a single
spanning tree (CST). The IST instance is simply an RSTP instance that
extends the CST inside the MST region.
The IST instance receives and sends BPDUs to the CST. The IST can
represent the entire MST region as a CST virtual bridge to the outside
It all makes a lot more sense with the diagrams.
Also see STP and MST:
IST, CIST, and CST Overview
Unlike other spanning tree protocols, in which all the spanning tree
instances are independent, MST establishes and maintains IST, CIST,
and CST spanning trees:
An IST is the spanning tree that runs in an MST region.
Within each MST region, MST maintains multiple spanning tree instances. Instance 0 is a special instance for a region, known as
the IST. All other MST instances are numbered from 1 to 4094.
The IST is the only spanning tree instance that sends and receives BPDUs. All of the other spanning tree instance information
is contained in MSTP records (M-records), which are encapsulated
within MST BPDUs. Because the MST BPDU carries information for all
instances, the number of BPDUs that need to be processed to support
multiple spanning tree instances is significantly reduced.
All MST instances within the same region share the same protocol timers, but each MST instance has its own topology
parameters, such as root bridge ID, root path cost, and so forth.
By default, all VLANs are assigned to the IST.
An MST instance is local to the region; for example, MST instance 1 in region A is independent of MST instance 1 in region
B, even if regions A and B are interconnected.
- A CIST is a collection of the ISTs in each MST region.
- The CST interconnects the MST regions and single spanning trees.
The spanning tree computed in a region appears as a subtree in the CST
that encompasses the entire switched domain. The CIST is formed by the
spanning tree algorithm running among switches that support the
802.1w, 802.1s, and 802.1D standards. The CIST inside an MST region is the same as the CST outside a region.
For more information, see the "Spanning Tree Operation Within an MST
Region" section and the "Spanning Tree Operations Between MST
Only the CST instance sends and receives BPDUs, and MST instances add
their spanning tree information into the BPDUs to interact with
neighboring switches and compute the final spanning tree topology.
Because of this, the spanning tree parameters related to BPDU
transmission (for example, hello time, forward time, max-age, and
max-hops) are configured only on the CST instance but affect all MST
instances. Parameters related to the spanning tree topology (for
example, switch priority, port VLAN cost, and port VLAN priority) can
be configured on both the CST instance and the MST instance.
MST switches use Version 3 BPDUs or 802.1D STP BPDUs to communicate
with 802.1D switches. MST switches use MST BPDUs to communicate with
IEEE 802.1s Terminology
Some MST naming conventions used in the prestandard implementation
have been changed to include identification of some internal and
regional parameters. These parameters are used only within an MST
region, compared to external parameters that are used throughout the
whole network. Because the CIST is the only spanning tree instance
that spans the whole network, only the CIST parameters require the
external qualifiers and not the internal or regional qualifiers.
- The CIST root is the root bridge for the CIST, which is the unique instance that spans the whole network.
- The CIST external root path cost is the cost to the CIST root. This cost is left unchanged within an MST region. Remember that an MST
region looks like a single switch to the CIST. The CIST external root
path cost is the root path cost calculated between these virtual
switches and switches that do not belong to any region.
- The CIST regional root was called the IST master in the prestandard implementation. If the CIST root is in the region, the CIST regional
root is the CIST root. Otherwise, the CIST regional root is the
closest switch to the CIST root in the region. The CIST regional root
acts as a root bridge for the IST.
- The CIST internal root path cost is the cost to the CIST regional root in a region. This cost is only relevant to the IST, instance 0.