Specifications
8-162 set stp port Rapier Switch Software Reference
Software Release 2.7.3
C613-03098-00 REV A
When STP mode is changed from standard to rapid, or rapid to standard, then
the pathcost parameter is mapped from one range to the other based on
relative deviation from the nearest default. We recommend that the pathcost
values be checked when changing mode to confirm that they are appropriate
for the network configuration.
If the pathcost of a port has not been explicitly set by the user or the defaults
have been restored to the port, then the default pathcost for the port varies as
the speed of the port varies.
IEEE Standard 802.1d, limited the range of the path cost parameter to a 16-bit
unsigned integer value. The recommended path cost values for rapid mode,
IEEE Standard 802.1w, make use of the full 32-bit range available in BPDUs.
The recommended values for an intermediate link speed can be calculated as
20000000000/(Link Speed in KB/s). This means that the accumulated Path
Cost values cannot exceed 32 bits over a concatenation of 20 hops. In LANs
where the recommended values defined in IEEE Standard 802.1d and IEEE
Standard 802.1w are required to interwork, one set of path cost values must be
reconfigured so that they are the same. The range of path costs that can be
configured in an older bridge is insufficient to accommodate the range of data
rates available.
The portpriority parameter sets the value of the priority field contained in the
port identifier. The Spanning Tree Algorithm uses the port priority when
determining the root port for each switch. The port with the lowest value is
considered to have the highest priority. The default is 128. Each STP has its
own independent portpriority parameter for each member port.
Table 8-31: Path cost values and port speed for standard mode
Port Speed Default pathcost Recommended pathcost
range
10Mbps 100 50 - 600
100Mbs 19 10 -60
1Gbps 4 3 -10
Table 8-32: Path cost values and port speed for rapid mode
Port Speed Default pathcost Recommended pathcost
range
Less than 100 Kb/s 200000000 20000000-200000000
1Mbps 20000000 2000000-20000000
10Mbps 2000000 200000-2000000
100 Mbps 200000 20000-200000
1 Gbps 20000 2000-20000
10 Gbps 2000 200-2000
100 Gbps 200 20-200
1Tbps 20 2-200
10 Tbps 2 2-20