Allied Telesis AT-S62 - Manual (Page 516)

Chapter 23: Multiple Spanning Tree Protocol
516 Section IV: Spanning Tree Protocols
Common and Internal Spanning Tree (CIST)
MSTP has a default spanning tree instance called the Common and
Internal Spanning Tree (CIST). This instance has an MSTI ID of 0.
This instance has unique features and functions that make it different from
the MSTIs that you create yourself. First, you cannot delete this instance
and you cannot change its MSTI ID.
Second, when you create a new port-based or tagged VLAN, it is by
default associated with the CIST and is automatically given an MSTI ID of
0. The Default_VLAN is also associated by default with CIST.
Another critical difference is that when you assign a VLAN to another
MSTI, it still partially remains a member of CIST. This is because CIST is
used by MSTP to communicate with other MSTP regions and with any
RSTP and STP single-instance spanning trees in the network. MSTP uses
CIST to participate in the creation of a spanning tree between different
regions and between regions and single-instance spanning tree, to form
one spanning tree for the entire bridged network.
The reason MSTP uses CIST to form the spanning tree of an entire
bridged network is because CIST can cross regional boundaries, while a
MSTI cannot. If a port is a boundary port, that is, if it is connected to
another region, that port automatically belongs solely to CIST, even if it
was assigned to an MSTI, because only CIST is active outside of a region.
As mentioned earlier, every MSTI must have a root bridge, referred to as a
regional root, in order to locate loops within the instance. CIST must also
have a regional root. However, the CIST regional root communicates with
the other MSTP regions and single-instance spanning trees in the bridged
network.
The CIST regional root is set with the CIST Priority parameter. This
parameter, which functions similar to the RSTP bridge priority value, is
used to select the root bridge for the entire bridged network. If an AT-8500
Series switch has the lowest CIST Priority value among all the spanning
tree bridges, it functions as the root bridge for all the MSTP regions and
STP and RSTP single-instance spanning trees in the network.