Load Sharing Using Stp Path Cost; Configuring Stp Path Costs And Load Sharing - Cisco WS-C3550-12G Software Configuration Manual

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
follows:
•
•
•
•
Figure 9-6
VLANs 2 – 4 (path cost 30)
VLANs 8 – 10 (path cost 19)

Configuring STP Path Costs and Load Sharing

Beginning in privileged EXEC mode, follow these steps to configure the network shown in
Command
Step 1
configure terminal
Step 2
interface fastethernet 0/1
Step 3
switchport trunk encapsulation
{isl | dot1q | negotiate}
Step 4 switchport mode trunk
Step 5
exit
Step 6
Step 7 end
Catalyst 3550 Multilayer Switch Software Configuration Guide
9-32
9-6, Trunk ports 1 and 2 are 100BASE-T ports. The path costs for the VLANs are assigned as
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.
Load-Sharing Trunks with Traffic Distributed by Path Cost
Trunk port 1
Switch 2
Switch 1
Trunk port 2
VLANs 8 – 10 (path cost 30)
VLANs 2 – 4 (path cost 19)
Purpose
Enter global configuration mode on Switch 1.
Enter interface configuration mode, and define Fast Ethernet port 0/1 as
the interface to be configured as a trunk.
Configure the port to support ISL or 802.1Q encapsulation.
You must configure each end of the link with the same encapsulation
type.
Configure the port as a trunk port.
The trunk defaults to ISL trunking.
Return to global configuration mode.
Repeat Steps 2 through 4 on Switch 1 interface Fast Ethernet 0/2.
Return to privileged EXEC mode.
Chapter 9 Creating and Maintaining VLANs
Figure 9-6:
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