Spanning Tree Introduction: STP, RSTP, MSTP
©2008 Allied Telesis Inc. All rights reserved.
18.14
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
Software Version 5.2.1
Common and Internal Spanning Tree (CIST)
The CIST is the default spanning tree instance of MSTP, i.e. all VLANs that are not members of
particular MSTIs are members of the CIST. Also, an individual MST region can be regarded as a
single virtual bridge by other MST regions. The spanning tree that runs between regions is the
CIST. The CIST is also the spanning tree that runs between MST regions and Single Spanning
Tree (SST) entities. So, in
Figure 18-3
, the STP that is running between the regions, and to the
SST bridges, is the CIST.
Figure 18-3:
The CIST operates on links between regions and to SST devices
Compatibility
with Previous
Spanning Tree
Protocols
MSTP provides for compatibility with older spanning tree protocols in several ways. In addition
to the MST region described in the previous section, the protocol provides for single spanning
tree systems by employing a Common and Internal Spanning Tree (CIST). The CIST applies a
common and internal spanning tree protocol to the whole of the bridged network and is a
direct equivalent to the internal spanning tree (IST) protocol of earlier versions.
In common with legacy spanning tree systems, the CIST protocol first determines its root
bridge from all the bridges on the network. This is the bridge that contains the lowest bridge
identifier. The protocol then selects a regional root bridge for each MSTR. This is the bridge
that provides the best path to the CIST root. After the MSTR root bridges have been chosen,
they then act on the region’s behalf in such a way that the region appears to the Common
Spanning Tree (CST) as a virtual bridge. So in addition to having multiple MSTIs, each region
operates as a bridge in a CST.
RSTP operates on these links
MSTP
Region 1
MSTP
Region 2
MSTP
Region 3
The three switches shown
are non-MSTP capable
MSTP_vs_RSTP