Unlike Rapid PVST+, in which all the STP instances are independent,
MST establishes and maintains IST, CIST, and CST spanning trees, as follows:
MST establishes and maintains additional spanning trees within each
MST region; these spanning trees are called multiple spanning tree instances
Instance 0 is a special instance for a region, known as the IST. The
IST always exists on all ports; you cannot delete the IST, or instance 0. By
default, all VLANs are assigned to the IST. All other MST instances are
numbered from 1 to 4094.
The IST is the only STP instance that sends and receives BPDUs. All of
the other MSTI information is contained in MST records (M-records), which are
encapsulated within MST BPDUs.
All MSTIs within the same region share the same protocol timers, but
each MSTI has its own topology parameters, such as the root bridge ID, the root
path cost, and so forth.
An MSTI is local to the region; for example, MSTI 9 in region A is
independent of MSTI 9 in region B, even if regions A and B are interconnected.
The CST interconnects the MST regions and any instance of 802.1D
and 802.1w STP that may be running on the network. The CST is the one STP
instance for the entire bridged network and encompasses all MST regions and
802.1w and 802.1D instances.
A CIST is a collection of the ISTs in each MST region. The CIST is
the same as an IST inside an MST region, and the same as a CST outside an MST
The spanning tree computed in an MST 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.