C
HAPTER
4
| Configuring the Switch
Configuring the Spanning Tree Algorithm
– 79 –
◆ Path Cost – This parameter is used by the STA to determine the best
path between devices. Therefore, lower values should be assigned to
ports attached to faster media, and higher values assigned to ports
with slower media. (Path cost takes precedence over port priority.)
By default, the system automatically detects the speed and duplex
mode used on each port, and configures the path cost according to the
values shown below.
◆ Priority – Defines the priority used for this port in the Spanning Tree
Protocol. If the path cost for all ports on a switch are the same, the port
with the highest priority (i.e., lowest value) will be configured as an
active link in the Spanning Tree. This makes a port with higher priority
less likely to be blocked if the Spanning Tree Protocol is detecting
network loops. Where more than one port is assigned the highest
priority, the port with lowest numeric identifier will be enabled.
(Range: 0-240, in steps of 16; Default: 128)
Table 5: Recommended STA Path Cost Range
Port Type IEEE 802.1D-1998 IEEE 802.1w-2001
Ethernet 50-600 200,000-20,000,000
Fast Ethernet 10-60 20,000-2,000,000
Gigabit Ethernet 3-10 2,000-200,000
Table 6: Recommended STA Path Costs
Port Type Link Type IEEE 802.1D-
1998
IEEE 802.1w-2001
Ethernet Half Duplex
Full Duplex
Trunk
100
95
90
2,000,000
1,999,999
1,000,000
Fast Ethernet Half Duplex
Full Duplex
Trunk
19
18
15
200,000
100,000
50,000
Gigabit Ethernet Full Duplex
Trunk
4
3
10,000
5,000
Table 7: Default STA Path Costs
Port Type Link Type IEEE 802.1w-2001
Ethernet Half Duplex
Full Duplex
Trunk
2,000,000
1,000,000
500,000
Fast Ethernet Half Duplex
Full Duplex
Trunk
200,000
100,000
50,000
Gigabit Ethernet Full Duplex
Trunk
10,000
5,000
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