Cisco Systems 3550 Load Sharing Using STP Path Cost

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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter9 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.
Figure9-6 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 configur e the netw ork sh own in Fi gure 9-6:
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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
Step1 configure terminal Enter global configuration mode on Switch 1.
Step2 interface fastethernet 0/1 Enter interface configuration mode, and define Fast Ethernet port 0/1 as
the interface to be configured as a trunk.
Step3 switchport trunk encapsulation
{isl | dot1q | negotiate}Configure the port to support ISL or 802.1Q encapsu lati on.
You must configure each end of the link with the same encapsulation
type.
Step4 switchport mode trunk Configure the port as a trunk port.
The trunk defaults to ISL trunking.
Step5 exit Return to global configuration mode.
Step6 Repeat Steps 2 through 4 on Switch 1 interface Fast Ethernet 0/ 2.
Step7 end Return to privileged EXEC mode.