Configuring Port Path Cost; Path Cost For Link Aggregate Ports - Alcatel OmniSwitch 6800 Series Network Configuration Manual

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.
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:
Link Speed
4 Mbps
10 Mbps
16 Mbps
100 Mbps
1 Gbps
10 Gbps
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. However, the port path cost is associated with the flat mode instance 0,
even though the physical 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:
OmniSwitch 6800 Series Network Configuration Guide
Recommended Recommended
Value
250 100–1000
100
62
19
4
2
Range
Range
1–65535
50–600 1–65535
40–400 1–65535
10–60 1–65535
3–10 1–65535
1–5 1–65535
November 2004
Configuring STP Port Parameters
page 5-17