Cisco 3845 Software Guide - Page 222

Configuring VLAN Trunks Chapter 10 Configuring VLANs Step 4 Step 5 Step 6 Command end show interfaces interface-id switchport copy running-config startup-config Purpose Return to privileged EXEC mode. Verify your entries in the Trunking Native Mode VLAN field. (Optional) Save your entries in the configuration file. 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 sending 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 that connect 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 the VLAN to which the traffic belongs. You configure load sharing on trunk ports that have STP enabled 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 13, "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 STP 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 10-2 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. • 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. 10-14 Cisco ME 3800X and 3600X Switch Software Configuration Guide OL-23400-01