Cisco Systems 3550 Load Sharing Using STP Port Priorities, Configuring STP Port Priorities and Load Sharing

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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 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 a blocking state. You can set the priorities on a parallel trunk port so that
the port carries all the traffic for a given VLAN. T he t ru nk po rt wi th t h e hi gh er pr i orit y (low er va lues)
for a VLAN is forwarding traffic for that VLAN. The trunk port wit h the lower priority (higher values)
for the same VLAN remains in a blocking state for that VLAN. One trunk port sends or receives all
traffic for the VLAN.
Figure 9-5 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 o f 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 f o r
all of the VLANs. No duplication of traffic occurs ove r an y tr unk po rt.
Figure9-5 Load Sharing by Using STP Port Priorities
Configuring STP Port Priorities and Load Sharing
Beginning in privileged EXEC mode, follow these steps to configur e the netw ork sh own in Fi gure 9-5:
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Switch 1
Switch 2
Trunk 2
VLANs 3 – 6 (priority 10)
VLANs 8 – 10 (priority 128)
Trunk 1
VLANs 8 – 10 (priority 10)
VLANs 3 – 6 (priority 128)
Command Purpose
Step1 vlan database On Switch 1, enter VLAN configuration mode.
Step2 vtp domain domain-name Configure a VTP administrative domain.
The domain name can be from 1 to 32 characters.
Step3 vtp server Configure Switch 1 as the VTP server.
Step4 exit Update the VLAN database, propagate it throughout the
administrative domain, and return to privileged EXEC m ode.
Step5 show vtp status Verify the VTP configuration on both Switch 1 and Switch 2.
In the display, check the VTP Operating Mode and the VTP Domain
Name fields.