Cisco 3560G 48TS Software Configuration Guide - Page 268

Configuring VLAN Trunks Chapter 12 Configuring VLANs 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 Chapter 15, "Configuring STP." Load Sharing Using STP Port Priorities When two ports on the same switch form a loop, the STP port priority setting determines 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. 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. Figure 12-3 Load Sharing by Using STP Port Priorities Switch A Trunk 1 VLANs 8 - 10 (priority 16) VLANs 3 - 6 (priority 128) Switch B Trunk 2 VLANs 3 - 6 (priority 16) VLANs 8 - 10 (priority 128) 93370 12-24 Catalyst 3560 Switch Software Configuration Guide 78-16156-01