HP ProCurve Series 2810 Management And Configuration Manual page 105

Note on Path Cost
RSTP and MSTP implements a greater range of path costs and new default
path cost values to account for higher network speeds. These values are
different than the values defined by 802.1D STP as shown below.
Port Type 802.1D STP Path Cost
10 Mbps 100
100 Mbps 10
1 Gbps 5
Because the maximum value for the path cost allowed by 802.1D STP is 65535,
devices running that version of spanning tree cannot be configured to match
the values defined by MSTP, at least for 10 Mbps and 100 Mbps ports. In LANs
where there is a mix of devices running 802.1D STP, RSTP, and/or MSTP, you
should reconfigure the devices so the path costs match for ports with the same
network speeds.
Multiple Instance Spanning-Tree Operation
RSTP and MSTP Path Cost
2 000 000
200 000
20 000
Overview
5-5