Configuring Trunk Ports For Load Sharing; Load Sharing Using Stp Port Priorities - Cisco WS-C3750-48PS-S Software Configuration Manual

Configuring VLAN Trunks
Command
Step 3 switchport trunk native vlan vlan-id
Step 4 end
Step 5
show interfaces interface-id switchport
Step 6 copy running-config startup-config
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 connecting 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

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
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.
Catalyst 3750 Switch Software Configuration Guide
13-24
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.
Purpose
Configure the VLAN that is sending and receiving untagged traffic
on the trunk port.
For vlan-id, the range is 1 to 4094.
Return to privileged EXEC mode.
Verify your entries in the Trunking Native Mode VLAN field.
(Optional) Save your entries in the configuration file.
Chapter 17, "Configuring STP."
Chapter 13 Configuring VLANs
78-16180-02