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Configuring STP Port Parameters
Configuring Spanning Tree Parameters
page 5-16
OmniSwitch 6624/6648 Network Configuration Guide
April 2004
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.
If the path cost is set to zero, then a default value based on link speed is used. The IEEE 802.1D standard
provides the following recommended values for path cost:
By default, STP is enabled on a port and the path cost is set to zero. If the switch is running in the 1x1
Spanning Tree mode, then the port path cost applies to the specified VLAN STP 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. VLAN 1 is referenced as the port’s VLAN, even if the port is associated
with other VLANs.
To change the path cost for a port, enter
bridge
followed by an existing VLAN ID (or VLAN 1 if using a
flat Spanning Tree instance) then the port’s
slot/port
designation followed by
path cost
and a numerical
value between 0 and 65535 for the path cost value. For example, the following command sets the path cost
for port 1 on slot 8 for VLAN 10 to 19 (the port speed is 100 Mbps, 19 is the recommended value).
-> bridge 10 8/1 path cost 19
Path Cost for Link Aggregate Ports
Physical ports that belong to a link aggregate do not participate in the Spanning Tree Algorithm. Instead,
the algorithm is applied to the aggregate logical link (virtual port) that represents a collection of physical
ports. By default, STP is enabled on the aggregate logical link and the path cost value is set to zero.
If the path cost for a link aggregate is set to zero, the following default values used are based on link speed
and link aggregate size. Note that for Gigabit ports the aggregate size is not applicable in this case:
Link Speed
Recommended
Value
Recommended
Range
Range
4 Mbps
250
100–1000
1–65535
10 Mbps
100
50–600
1–65535
16 Mbps
62
40–400
1–65535
100 Mbps
19
10–60
1–65535
1 Gbps
4
3–10
1–65535
10 Gbps
2
1–5
1–65535