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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 9 Creating and Maintaining VLANs
Understanding VLAN Trunks
Load Sharing Using STP Path Cost
You can configure parallel trunks to share VLAN traffic by setting different path costs on a trunk and
associating the path costs with different sets of VLANs. The VLANs keep the traffic separate. Because
no loops exist, STP does not disable the ports, and redundancy is maintained in the event of a lost link.
In Figure 9-6, Trunk ports 1 and 2 are 100BASE-T ports. The path costs for the VLANs are assigned as
follows:
• VLANs 2 through 4 are assigned a path cost of 30 on Trunk port 1.
• VLANs 8 through 10 retain the default 100BASE-T path cost on Trunk port 1 of 19.
• VLANs 8 through 10 are assigned a path cost of 30 on Trunk port 2.
• VLANs 2 through 4 retain the default 100BASE-T path cost on Trunk port 2 of 19.
Figure 9-6 Load-Sharing Trunks with Traffic Distributed by Path Cost
Configuring STP Path Costs and Load Sharing
Beginning in privileged EXEC mode, follow these steps to configure the network shown in Figure 9-6:
16591
Switch 1
Switch 2
Trunk port 1
VLANs 2 – 4 (path cost 30)
VLANs 8 – 10 (path cost 19)
Trunk port 2
VLANs 8 – 10 (path cost 30)
VLANs 2 – 4 (path cost 19)
Command Purpose
Step 1
configure terminal Enter global configuration mode on Switch 1.
Step 2
interface fastethernet 0/1 Enter interface configuration mode, and define Fast Ethernet port 0/1 as
the interface to be configured as a trunk.
Step 3
switchport trunk encapsulation
{isl | dot1q | negotiate}
Configure the port to support ISL or 802.1Q encapsulation.
You must configure each end of the link with the same encapsulation
type.
Step 4
switchport mode trunk Configure the port as a trunk port.
The trunk defaults to ISL trunking.
Step 5
exit Return to global configuration mode.
Step 6
Repeat Steps 2 through 4 on Switch 1 interface Fast Ethernet 0/2.
Step 7
end Return to privileged EXEC mode.