Allied Telesis AT-GS950/24 - Manual (Page 379)

AT-GS950/24 Web Interface User Guide
379
 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 Bridge Priority value and a bridge’s MAC
address.
 The regional root of an 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 or
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.
MSTP uses CIST to form the spanning tree of an entire bridged network
because CIST can cross regional boundaries, while an 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-GS950/24 switch has
the lowest CIST Priority value among all the spanning tree bridges, it functions
as the root bridge for all of 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-GS950/24
switch running MSTP receives STP BPDUs, the port only sends 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.