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Configuring Spanning Tree Parameters
Configuring Port Path Cost
The path cost value specifies the contribution of a port to the path cost towards the root bridge that
includes the port. The root path cost is the sum of all path costs along this same path and is the value
advertised in Configuration BPDU transmitted from active Spanning Tree ports. The lower the cost value,
the closer the switch is to the root.
The type of path cost value used depends on which path cost mode is active (automatic or 32-bit). If the
path cost mode is set to automatic, a 16-bit value is used when STP or RSTP is the active protocol and a
32-bit value is used when MSTP is the active protocol. If the mode is set to 32-bit, then a 32-bit path cost
value is used regardless of which protocol is active. See
for more information.
If a 32-bit path cost value is in use and the path_cost is set to zero, the following IEEE 802.1Q 2005
recommended default path cost values based on link speed are used:
Link Speed
Recommended Value
10 MB
100 MB
1 GB
10 Gbps
Is a 16-bit path cost value is in use and the path_cost is set to zero, the following IEEE 802.1D
recommended default path cost values based on link speed are used:
Link Speed
Recommended Value
4 Mbps
10 Mbps
16 Mbps
100 Mbps
1 Gbps
10 Gbps
Spanning Tree is automatically enabled on a port and the path cost is set to the default value. If the switch
is running in the per-VLAN Spanning Tree mode, then the port path cost applies to the specified VLAN
instance associated with the port. If the switch is running in the flat Spanning Tree mode, then the port
path cost applies across all VLANs associated with the port. The flat mode instance is specified as the port
instance, even if the port is associated with other VLANs.
The
spantree vlan path-cost
switch is running in either mode (per-VLAN or flat). For example, the following command configures a
16-bit path cost value for port 8/1 for VLAN 10 to 19 (the port speed is 100 MB, 19 is the
recommended value):
-> spantree vlan 10 port 8/1 path-cost 19
To change the port path cost value for the flat mode instance regardless of which mode (per-VLAN or flat)
is active for the switch, use the
OmniSwitch AOS Release 7 Network Configuration Guide
IEEE 802.1D
2,000,000
200,000
20,000
2,000
IEEE 802.1D
250
100
62
19
4
2
command configures the port path cost value for a VLAN instance when the
spantree cist path-cost
Configuring STP Port Parameters
"Configuring the Path Cost Mode" on page 5-31
command. For example, the following command
March 2011
page 5-37
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