Spanning Tree Path Cost Support for 802.1D 2004

Support for the IEEE 802.1D 2004 defined set of spanning tree path costs can be configured globally for high-speed ports.

To ensure a loop-free topology, Spanning Tree Protocol (STP) determines a single active path between two network nodes by calculating a path cost from various properties, including the bridge ID, bridge priority, network segment cost, and port priority. The original IEEE 802.1D 1998 standard allows for a 16-bit path cost value. The introduction of higher port speeds such as 40 Gbps up to 10 Tbps led to a new IEEE 802.1D 2004 standard that allows a 32-bit path cost value for more granularity in assigning path costs using a redefined path cost formula.

STP Path Cost = 20,000,000,000/Link speed in Kbps

The challenge is to have all bridge devices in the network running the same set of path costs. A command line interface (CLI) command, spanning-tree path-cost-method with two keyword options of long or short, has been created to globally change the STP path cost method for all ports running STP. The 802.1D 1998 set of path cost options is the default setting and is referred to as the short option. When configured, the long option upgrades all ports running STP to use the 802.1D 2004 defined set of spanning tree path costs. To revert to the 802.1D 1998 set of path cost options, use the short option in global configuration mode.

Note: Any path cost configured for an individual interface is overwritten when the STP path costs are configured globally.
Note: LAG formation follows the legacy rule where aggregation of ports is allowed only for ports with the same default speed and media type.