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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 8 Configuring VLANs
Load Sharing Using STP
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
sharingusingSTPpathcosts,eachload-sharinglinkcanbeconnectedtothesameswitchortotwo
different switches.
For more information about STP, see the “Understanding Basic STP Features” section on page 9-1.
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 standby mode. 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 transmits or receives all traffic
for the VLAN.
Figure 8-4 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 10 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 10 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 8-4 Load Sharing by Using STP Port Priorities
Configuring STP Port Priorities and Load Sharing
Beginning in privileged EXEC mode, follow these steps to configure the network shown in Figure 8-4:
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Switch 1
Switch 2
Trunk 2
VLANs 3 – 6 (priority 10)
VLANs 8 – 10 (priority 128)
Trunk 1
LANs 8 – 10 (priority 10)
LANs 3 – 6 (priority 128)
Command Purpose
Step 1
vlan database On Switch 1, enter VLAN configuration mode.
Step 2
vtp domain
domain-name
Configure a VTP administrative domain.
Thedomainnamecanbefrom1to32characters.
Step 3
vtp server Configure Switch 1 as the VTP server.
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