Allied Telesis AT-S63 - Manual (Page 244)

Chapter 22: Multiple Spanning Tree Protocol
244 Section V: 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. Firstly, you cannot delete this instance
and you cannot change its MSTI ID.
Secondly, 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.
MSTP uses CIST to form the spanning tree of an entire bridged network
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,
selects the root bridge for the entire bridged network. If the AT-9400
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.
MSTP with STP
and RSTP
MSTP is fully compatible with STP and RSTP. If a port on the AT-9400
Switch running MSTP receives STP BPDUs, the port sends only STP
BPDU packets. If a port receives RSTP BPDUs, the port sends MSTP
BPDUs because RSTP can process MSTP BPDUs.
A port connected to a bridge running STP or RSTP is considered to be a
boundary port of the MSTP region and the bridge as belonging to a
different region.
An MSTP region can be considered as a virtual bridge. The implication is
that other MSTP regions and STP and RSTP single-instance spanning
trees cannot discern the topology or constitution of a MSTP region. The
only bridge they are aware of is the regional root of the CIST instance.