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Example VLAN Trunk Configurations
Load-Sharing VLAN Traffic Over Parallel Trunks Example
Using spanning-tree port-VLAN priorities, you can load-share VLAN traffic over parallel trunk ports so
that traffic from some VLANs travels over one trunk, while traffic from other VLANs travels over the
other trunk. This configuration allows traffic to be carried over both trunks simultaneously (instead of
keeping one trunk in blocking mode), which reduces the total traffic that is carried over each trunk while
still maintaining a fault-tolerant configuration.
Figure 5-3
ports on the supervisor engine.
Figure 5-3
VLANs 10, 20, 30, 40, 50, and 60: port-VLAN priority 32 (blocking)
1/1
VLANs 10, 20, 30, 40, 50, and 60: port-VLAN priority 32 (forwarding)
By default, the port-VLAN priority for both trunks is equal (a value of 32). STP blocks port 1/2
(Trunk 2) for each VLAN on Switch 1 to prevent forwarding loops. Trunk 2 is not used to forward traffic
unless Trunk 1 fails.
To configure the switches so that traffic from multiple VLANs is load balanced over the parallel trunks,
perform these steps:
Configure a VTP domain on both Switch 1 and Switch 2 by entering the set vtp command so that the
Step 1
VLAN information that is configured on Switch 1 is learned by Switch 2. Make sure that Switch 1 is a
VTP server. You can configure Switch 2 as a VTP client or as a VTP server.
Switch_1> (enable) set vtp domain BigCorp mode server
VTP domain BigCorp modified
Switch_1> (enable)
Switch_2> (enable) set vtp domain BigCorp mode server
VTP domain BigCorp modified
Switch_2> (enable)
Create the VLANs on Switch 1 by entering the set vlan command. In this example, you see VLANs 10,
Step 2
20, 30, 40, 50, and 60.
Switch_1> (enable) set vlan 10
Vlan 10 configuration successful
Switch_1> (enable) set vlan 20
Vlan 20 configuration successful
Switch_1> (enable) set vlan 30
Vlan 30 configuration successful
Switch_1> (enable) set vlan 40
Vlan 40 configuration successful
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
5-22
shows a parallel trunk configuration between two switches using the Fast Ethernet uplink
Parallel Trunk Configuration Before Configuring VLAN-Traffic Load Sharing
Trunk 2
Switch 1
1/2
Trunk 1
Chapter 5
Configuring Ethernet VLAN Trunks
Switch 2
1/1
1/2
OL-8978-04
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