Load Sharing Using Stp Port Priorities - Cisco Catalyst 3750 Software Configuration Manual

Chapter 10 Configuring VLANs

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 10-3
configured as follows:
•
•
•
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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 10-3 Load Sharing by Using STP Port Priorities
VLANs 8 – 10 (priority 16)
VLANs 3 – 6 (priority 128)
Beginning in privileged EXEC mode, follow these steps to configure the network shown in
Command
Step 1
configure terminal
Step 2 vtp domain domain-name
Step 3 vtp mode server
Step 4
end
Step 5 show vtp status
Step 6 show vlan
Step 7
configure terminal
78-15870-01
shows two trunks connecting supported switches. In this example, the switches are
VLANs 8 through 10 are assigned a port priority of 16 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 16 on Trunk 2.
VLANs 8 through 10 retain the default port priority of 128 on Trunk 2.
Trunk 1
Switch B
Switch A
Trunk 2
VLANs 3 – 6 (priority 16)
VLANs 8 – 10 (priority 128)
Purpose
Enter global configuration mode on Switch A.
Configure a VTP administrative domain.
The domain name can be from 1 to 32 characters.
Configure Switch A as the VTP server.
Return to privileged EXEC mode.
Verify the VTP configuration on both Switch A and Switch B.
In the display, check the VTP Operating Mode and the VTP Domain
Name fields.
Verify that the VLANs exist in the database on Switch A.
Enter global configuration mode.
Catalyst 3750 Metro Switch Software Configuration Guide
Configuring VLAN Trunks
Figure 10-3.
10-25