Appendix A: MSTP Overview
342
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 DefaultVLAN 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-GS950/24 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-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.
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.
