With PVST and Rapid PVST, the 802.1D-2004 costs can be activated using the spanning-tree pathcost method long global configuration command. By default, spanning-tree pathcost method short is configured, causing the switch to use the older revision of the costs.
- Chapter 3. Spanning Tree Protocol
- from CCIE Routing and Switching v5.0 Official Cert Guide, Volume 1, Fifth Edition
- Publisher: Cisco Press
- Released: August 2014
The path cost is the metric STP uses to calculate the shortest path to the elected root bridge. The path cost is based on the speed of the bridge port interface. if you change your network to using the "long mode" then it must be changed universally within the LAN-switched environment. All of your switches should agree upon the method of spanning tree, the timers, and the path cost metrics. If not, then you could have inconsistencies that could cause your spanning tree not to converge properly as you would expect.
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