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Multiple Instance Spanning-Tree Operation
802.1s Multiple Spanning Tree Protocol (MSTP)
In a mesh environment, the default MSTP timer settings (Hello Time and
Forward Delay) are usually adequate for MSTP operation. Because a packet
crossing a mesh may traverse several links within the mesh, using smaller-
than-default settings for the MSTP Hello Time and Forward Delay timers can
cause unnecessary topology changes and end-node connectivity problems.
For MSTP information beyond what is provided in this manual, refer to the
IEEE 802.1s standard.
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.
Figure 4-2. Example of MSTP Network with Legacy STP and RSTP Devices
Connected
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)