Allied Telesis AT-S60 - Manual (Page 267)

AT-S60 Management Software User’s Guide
Section II: Advanced Features 267
❑ Each MSTI must have a regional root for locating loops in the
instance. MSTIs can share the same regional root or have different
roots. A regional root is determined by the MSTI priority value and
a bridge’s MAC address.
❑ The regional root of a MSTI must be in the same region as the
MSTI.
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 that might exist 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-8400 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.