Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 484)

Spanning Tree Introduction: STP, RSTP, and MSTP
Software Reference for x310 Series Switches
18.16 AlliedWare Plus TM Operating System - Version 5.4.4C C613-50046-01 REV A
CIST In addition to the individual MSTIs within each MSTP region, the MSTP region is a member
of a network-wide spanning tree called the Common and Internal Spanning Tree (CIST).
Conceptually, each region represents a virtual bridge. Internal and external bridge
connectivity are two independent functions.
Frames with VIDs allocated to the CIST are subject to the rules and path costs of the
complete bridged LAN as determined by the CIST’s vectors. Frames other than these are
subject to the CIST when traveling outside their region, and subject to its particular MSTI
inside the region.
The following operational rules apply:
■ Each bridge can be a member of only one region.
■ A data frame is associated with a single VID.
■ Data frames with a given VID are associated with either the CIST or their particular
MSTI, but not both.
The role of the Common Spanning Tree (CST) in a network, and the Common and Internal
Spanning Tree (CIST) configured on each device, is to prevent loops within a wider
network that may span more than one MSTP region and parts of the network running in
legacy STP or RSTP mode.
CIST first allocates root and designated bridges by selecting the bridge with the lowest
identifier as the root. MSTP then deals with any loops between the regions in the CST. It
does this by considering the CIST “vectors” in the following order:
1. CIST External Root Path Cost
2. CIST Regional Root Identifier
3. CIST Internal Root Path Cost
4. CIST Designated Bridge Identifier
5. CIST Designated Port Identifier
6. CIST Receiving Port Identifier