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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 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. The trunk port with the higher priority (lower values)
for a VLAN is forwarding traffic for that VLAN. The trunk port with 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 of 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 for
all of the VLANs. No duplication of traffic occurs over any trunk port.
Figure 9-5
Load Sharing by Using STP Port Priorities
Configuring STP Port Priorities and Load Sharing
Beginning in privileged EXEC mode, follow these steps to configure the network shown in
Figure 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
Step 1
vlan database
On Switch 1, enter VLAN configuration mode.
Step 2
vtp domain domain-name
Configure a VTP administrative domain.
The domain name can be from 1 to 32 characters.
Step 3
vtp server
Configure Switch 1 as the VTP server.
Step 4
exit
Update the VLAN database, propagate it throughout the
administrative domain, and return to privileged EXEC mode.
Step 5
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.
