Spanning-Tree Operation
802.1s Multiple Spanning Tree Protocol (MSTP)
MSTP Structure
MSTP maps active, separate paths through separate spanning tree instances
and between MST regions. Each MST region comprises one or more MSTP
switches. Note that MSTP recognizes an STP or RSTP LAN as a distinct
spanning-tree region.
IST
Instance
MSTI
(Optional)
MSTI
(Optional)
IST
Instance
MSTI
(Optional)
MSTI
(Optional)
MSTI
(Optional)
Switch
Running STP
Switch
Running STP
Switch
Running STP
Switch
Running RSTP
Switch
Running RSTP
Switch
Running RSTP
Common Spanning Tree (CST)
MST Region
MST Region
Common and Internal Spanning Tree (CIST)
Figure 6-25. Example of MSTP Network with Legacy STP and RSTP Devices
Connected
Common and Internal Spanning Tree (CIST): The CIST identifies the
regions in a network and administers the CIST root bridge for the network,
the root bridge for each region, and the root bridge for each spanning-tree
instance in each region.
Common Spanning Tree (CST): The CST administers the connectivity
among the MST regions, STP LANs, and RSTP LANs in a bridged network.
6-45
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