Cisco Systems OL-8550-09 Load Sharing Using STP Port Priorities

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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter13 Configuring VLANs
Configuring VLAN Trunks
Load Sharing Using STP Port Priorities
When two ports on the same switch form a loop, the switch uses the STP port priority to decide 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 13-3 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 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.
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.
Figure13-3 Load Sharing by Using STP Port Priorities
Note If your switch is a member of a switch stack, you must use the spanning-tree [vlan vlan-id] cost cost
interface configuration command instead of the spanning-tree [vlan vlan-id] port-priority priority
interface configuration command to select an interface to put in the forwarding state. Assign lower cost
values to interfaces that you want selected first and higher cost values that you want selected last. For
more information, see the “Load Sharing Using STP Path Cost” section on page13-26.
Stacking is supported only on Catalyst 2960-S switches running the LAN base image.
Beginning in privileged EXEC mode, follow these steps to configure the network shown in Figure13-3.
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Switch A
Switch B
Trunk 2
VLANs 3 – 6 (priority 16)
VLANs 8 – 10 (priority 128)
Trunk 1
VLANs 8 – 10 (priority 16)
VLANs 3 – 6 (priority 128)
Command Purpose
Step1 configure terminal Enter global configuration mode on SwitchA.
Step2 vtp domain domain-name Configure a VTP administrative domain.
The domain name can be 1 to 32 characters.
Step3 vtp mode server Configure Switch A as the VTP server.
Step4 end Return to privileged EXEC mode.