Load Sharing Using Stp Port Priorities - Cisco Catalyst 3120 Software Manual

Chapter 12 Configuring VLANs
You configure load sharing on trunk ports by using STP port priorities or STP path costs. For load
sharing using STP port priorities, both load-sharing links must be connected to the same switch. For load
sharing using STP path costs, each load-sharing link can be connected to the same switch or to two
different switches. For more information about STP, see

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 12-4
configured as follows:
•
•
•
•
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 12-4
VLANs 8 – 10 (priority 16)
VLANs 3 – 6 (priority 128)
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
OL-12247-01
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.
Load Sharing by Using STP Port Priorities
Trunk 1
Switch B
"Load Sharing Using STP Path Cost" section on page
Chapter 17, "Configuring STP."
Switch A
Trunk 2
VLANs 3 – 6 (priority 16)
VLANs 8 – 10 (priority 128)
Cisco Catalyst Blade Switch 3120 for HP Software Configuration Guide
Configuring VLAN Trunks
12-27.
12-25