Introduction Spanning Tree Protocol (STP and RSTP)
Software Reference for SwitchBlade x3100 Series Switches (Layer Two Switching)
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Note: To maintain compatibility for comparison with previous versions of STP, the port priority is now
considered to be a value between 0-240 that can only be provisioned in increments of 16.
4.5.3.9 Interface Path Costs
Interlace path costs are used in root path cost calculations, which are a factor in root interface and designated
bridge elections. By default, interface path costs are related to the bandwidth capacity of the interfaces; how-
ever, the default values may be changed by the user to reflect other factors (e.g. propagation delay, link quality,
desired traffic level, etc.)
The default values and recommended ranges for path cost are as follows:
• Interface Speed: 10 Mbps
• Default Path Cost: 100
• Recommended Range: 50-600
• Interface Speed: 100 Mbps
• Default Path Cost: 10
• Recommended Range: 10-60
• Interface Speed: 1 Gbps
• Default Path Cost: 20
• Recommended Range: 3-10
The path cost values identified above reflect what is implemented in the initial SBx3112 product release as iden-
tified in IEEE Std.802.1d, 1998 Edition. The corresponding default values and recommended ranges for path
cost as specified in IEEE Std. 802.1w-2001 to support RSTP and MSTP are shown in the table below.
• Interface Speed: 10 Mbps
• Default Path Cost: 2,000,000
• Recommended Range: 200,000-20,000,000
• Interface Speed: 100 Mbps
• Default Path Cost: 200,000
• Recommended Range: 20,000-2,000,000
• Interface Speed: 1 Gbps
• Default Path Cost: 20,000
• Recommended Range: 2,000-200,000
A calculation is shown below that can be used to determine the recommended path cost value to use for inter-
mediate link speeds:
20,000,000,000 / (link speed in kb/s)
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