Allied Telesis AT-S63 - Manual (Page 146)

Chapter 8: Port Trunking
146 Section I: Basic Features
Note that packets sent back from the destination node to the original
source node may travel the same data link or a different data link in the
trunk.
As a general rule, the source address load distribution method is useful
in situations where the number of source nodes equals or is greater than
the number of data links in the trunk.
So when would the source address method be inappropriate? Returning
to the example in Figure 41 on page 145, assume that you configured
switch #2 also for source MAC address load distribution. The result
would be that the switch would use only one data link in the trunk to
send packets back to switch #1, because there is only one source, a
Gigabit Ethernet server. Because there is only one source, only one data
link is used. So obviously this method is not appropriate when there are
fewer source nodes than data links.
Destination Address Distribution Methods
The destination address method is much the same as the source address
method. The difference is that the destination address of a packet, rather
than its source address, is used to distribute the traffic across the ports of
a trunk.
When a switch receives a packet from a network node, it examines the
destination address to determine on which switch port, if any, the
packet should be transmitted. If the packet is destined for a port trunk
and if this is the first packet intended for that destination address to
cross the trunk, the switch assigns the destination address to one of the
trunk links.
Destination addresses are assigned to the ports of a trunk in a round-
robin fashion. If this is the first packet to be sent over the trunk, the
destination address is assigned to the lowest numbered port in the
trunk. All subsequent packets intended to the destination node are sent
out the assigned data link of the trunk.
When another node sends a packet over the trunk, its address is
assigned to the next lowest port in the trunk, and so forth. After an
address has been assigned to all the ports in the trunk, the process is
repeated starting with the lowest numbered port.
Destination address trunking is typically used in a situation where there
is one or just a few source nodes transmitting to many destination
nodes. Switch #2 in Figure 41 on page 145 is an example of where this
type of load distribution would be useful. The server connected to the
switch is sending packets to multiple destination nodes.