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| 3.5 |  | Evolution of Spanning Tree |
| 3.5.4 |  | Load balancing |

Load sharing can be accomplished using a couple of methods. The most common method of load sharing is through root bridge placement on a per-VLAN basis. This will distribute traffic for separate VLANs across separate paths to different root bridges. A separate method 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, traffic can be divided between the links according to which VLAN the traffic belongs.

Load sharing can be configured 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.

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. The priorities on a parallel trunk port can be set 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 shows two trunks connecting supported switches configured as follows:

- LANs 8 through 10 are assigned a port priority of 10 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 10 on Trunk 2.
- VLANs 8 through 10 retain the default port priority of 128 on Trunk 2.

In this configuration, 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.

Load Sharing Using STP Path Cost
Parallel trunks can be configured to share VLAN traffic by setting different path costs on a trunk and associating the path costs with different sets of VLANs. The VLANs keep the traffic separate. Because no loops exist, STP does not disable the ports, and redundancy is maintained in the event of a lost link.

Figure shows that Trunk ports 1 and 2 are 100BASE-T ports. The path costs for the VLANs are assigned as follows:

- VLANs 2 through 4 are assigned a path cost of 30 on Trunk port 1.
- VLANs 8 through 10 retain the default 100BASE-T path cost on Trunk port 1 of 19.
- VLANs 8 through 10 are assigned a path cost of 30 on Trunk port 2.
- VLANs 2 through 4 retain the default 100BASE-T path cost on Trunk port 2 of 19.

In this configuration, 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 higher Path Cost takes over and carries the traffic for all of the VLANs. No duplication of traffic occurs over any trunk port.