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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 17 Configuring VLANs
Configuring VLAN Trunks
Load Sharing Using STP
Load sharing divides the bandwidth supplied by parallel trunks connecting switches. To avoid loops,
STP normally blocks all but one parallel link between switches. Using load sharing, you divide the traffic
between the links according to which VLAN the traffic belongs.
You configure load sharing on trunk ports by using STP port priorities or STP path costs. For load
sharing using STP port priorities, both load-sharing links must be connected to the same switch. For load
sharing using STP path costs, each load-sharing link can be connected to the same switch or to two
different switches. For more information about STP, see Chapter 14, “Configuring STP.”
Load Sharing Using STP Port Priorities
When two ports on the same switch form a loop, the STP port priority setting determines which port is
enabled and which port is in a blocking state. You can set the priorities on a parallel trunk port so that
the port carries all the traffic for a given VLAN. The trunk port with the higher priority (lower values)
for a VLAN is forwarding traffic for that VLAN. The trunk port with the lower priority (higher values)
for the same VLAN remains in a blocking state for that VLAN. One trunk port sends or receives all
traffic for the VLAN.
Figure 17-3 shows two trunks connecting supported switches. In this example, the switches are
configured as follows:
• VLANs 8 through 10 are assigned a port priority of 16 on Trunk 1.
• VLANs 3 through 6 retain the default port priority of 128 on Trunk 1.
• VLANs 3 through 6 are assigned a port priority of 16 on Trunk 2.
• VLANs 8 through 10 retain the default port priority of 128 on Trunk 2.
In this way, Trunk 1 carries traffic for VLANs 8 through 10, and Trunk 2 carries traffic for VLANs 3
through 6. If the active trunk fails, the trunk with the lower priority takes over and carries the traffic for
all of the VLANs. No duplication of traffic occurs over any trunk port.
Figure 17-3 Load Sharing by Using STP Port Priorities
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Switch A
Switch B
Trunk 2
VLANs 3 – 6 (priority 16)
VLANs 8 – 10 (priority 128)
Trunk 1
VLANs 8 – 10 (priority 16)
VLANs 3 – 6 (priority 128)
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