Load Sharing Using Stp Port Priorities - Cisco ME 3400G-2CS - Ethernet Access Switch Software Configuration Manual

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 NNI trunk
port so that the port carries all the traffic for a given VLAN. The NNI trunk port with the higher priority
(lower values) for a VLAN is forwarding traffic for that VLAN. The NNI 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 11-3
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 11-3
VLANs 8 – 10 (priority 16)
VLANs 3 – 6 (priority 128)
Beginning in privileged EXEC mode on Switch A, follow these steps to configure the network shown in
Figure
Command
Step 8 show vlan
Step 9 configure terminal
Step 10
interface gigabitethernet 0/1
Step 11
port-type nni
Step 12 switchport mode trunk
Step 13 spanning-tree vlan 8-10 port-priority 16
Step 14
end
Step 15 show interfaces gigabitethernet 0/1
switchport
Step 16 configure terminal
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
11-20
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
11-3. Note that you can use any interface numbers; those shown are examples only.
Switch A
Trunk 2
VLANs 3 – 6 (priority 16)
VLANs 8 – 10 (priority 128)
Purpose
Verify that the referenced VLANs exist on Switch A. If not, create the
VLANs by entering the VLAN IDs.
Enter global configuration mode.
Define the interface to be configured as the Trunk 1 interface, and
enter interface configuration mode.
Configure the interface as an NNI. UNIs do not support STP.
Configure the port as a trunk port.
Assign the port priority of 16 for VLANs 8 through 10 on Trunk 1.
Return to privileged EXEC mode.
Verify the port configuration.
Enter global configuration mode.
Chapter 11 Configuring VLANs
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