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Catalyst 2960 and 2960-S Switch Software Configuration Guide
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Chapter 13 Configuring VLANs
Configuring VLAN Trunks
For information about IEEE 802.1Q configuration issues, see the “IEEE 802.1Q Configurat ion
Considerations” section on page 13-14.
Beginning in privileged EXEC mode, follow these steps to configure the native VLAN on an
IEEE 802.1Q trunk:
To return to the default native VLAN, VLAN 1, use the no switchport trunk native vlan interface
configuration command.
If a packet has a VLAN ID that is the same as the outgoing port native VLAN ID, the packet is sent
untagged; otherwise, the switch sends the packet with a tag.
Configuring Trunk Ports for Load Sharing
Load sharing divides the bandwidth supplied by parallel trunks connect ing switches. To avoid loops,
STP normally blocks all but one parallel link between switches. Using load sharing, you divide the traffic
between the links according to which VLAN the traffic belongs.
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 Chapter 16, “Configuring STP.”

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 po rt 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 13-2 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.
Command Purpose
Step 1 configure terminal Enter global configuration mode.
Step 2 interface interface-id Define the interface that is configured as the IEEE 802.1Q trunk, and
enter interface configuration mode.
Step 3 switchport trunk native vlan vlan-id Configure the VLAN that is sending and receiving untagged traffic
on the trunk port.
For vlan-id, the range is 1 to 4094.
Step 4 end Return to privileged EXEC mode.
Step 5 show interfaces interface-id switchport Verify your entries in the Trunking Native Mode VLAN field.
Step 6 copy running-config startup-config (Optional) Save your entries in the configuration file.