Illustration of MST Regions
This figure displays three MST regions and a legacy IEEE 802.1D switch (D). The CIST regional root for
region 1 (A) is also the CIST root. The CIST regional root for region 2 (B) and the CIST regional root for
region 3 (C) are the roots for their respective subtrees within the CIST. The RSTP runs in all regions.
Figure 12: MST Regions, CIST Masters, and CST Root
Related Topics
Multiple Spanning-Tree Regions, on page 245
Operations Within an MST Region, on page 246
Operations Between MST Regions, on page 246
Hop Count
The IST and MST instances do not use the message-age and maximum-age information in the configuration
BPDU to compute the spanning-tree topology. Instead, they use the path cost to the root and a hop-count
mechanism similar to the IP time-to-live (TTL) mechanism.
By using the spanning-tree mst max-hops global configuration command, you can configure the maximum
hops inside the region and apply it to the IST and all MST instances in that region. The hop count achieves
the same result as the message-age information (triggers a reconfiguration). The root switch of the instance
always sends a BPDU (or M-record) with a cost of 0 and the hop count set to the maximum value. When a
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
248
Information About MSTP
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