The two trunk group options (LACP and trunk) use SA/DA pairs for distributing outbound traffic over trunked links. That is, the switch sends traffic from the same source address to the same destination address through the same trunked link, and may also send traffic from the same source address to a different destination address through the same link or a different link, depending on the mapping of path assignments among the links in the trunk. Likewise, the switch distributes traffic for the same destination address but from different source addresses through links depending on the path assignment.

The load-balancing is done on a per-communication basis. Otherwise, traffic is transmitted across the same path as shown in Example: of single path traffic through a trunk. That is, if Client A attached to Switch 1 sends five packets of data to Server A attached to Switch 2, the same link is used to send all five packets. The SA/DA address pair for the traffic is the same. The packets are not evenly distributed across any other existing links between the two switches; they all take the same path.

The actual distribution of the traffic through a trunk depends on a calculation using bits from the SA/DA. When an IP address is available, the calculation includes the last five bits of the IP source address and IP destination address; otherwise, the MAC addresses are used. The result of that process undergoes a mapping that determines which link the traffic goes through. If you have only two ports in a trunk, it is possible that all the traffic will be sent through one port even if the SA/DA pairs are different. The more ports you have in the trunk, the more likely it is that the traffic will be distributed among the links.

When a new port is added to the trunk, the switch begins sending traffic, either new traffic or existing traffic, through the new link. As links are added or deleted, the switch redistributes traffic across the trunk group. For example, in Example: of port-trunked network showing a three-port trunk, traffic could be assigned as shown in Example: of link assignments in a trunk group (SA/DA distribution).

Because the amount of traffic coming from or going to various nodes in a network can vary widely, it is possible for one link in a trunk group to be fully utilized while other links in the same trunk have unused bandwidth capacity, even if the assignments were evenly distributed across the links in a trunk.