Common And Internal Spanning Tree (Cist); Mstp Regions - Allied Telesis SwitchBlade x908 Series Software Reference Manual

Spanning Tree Introduction: STP, RSTP, and MSTP

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
SST bridges, is the CIST.
Figure 18-3:
Compatibility MSTP provides for compatibility with older spanning tree protocols in several ways. In addition
with Previous to the MST region described in the previous section, the protocol provides for single spanning
Spanning Tree tree systems by employing a Common and Internal Spanning Tree (CIST). The CIST applies a
Protocols 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.
18.14
Figure 18-3, the STP that is running between the regions, and to the
The CIST operates on links between regions and to SST devices
MSTP
Region 1
MSTP
Region 2
Software Reference for SwitchBlade® x908, x900 and x600 Series Switches
TM
AlliedWare Plus Operating System - Software Version 5.3.1
MSTP
Region 3
The three switches shown
are non-MSTP capable
RSTP operates on these links
MSTP_vs_RSTP
C613-50007-01 REV B