Allied Telesis AT-S62 - Manual (Page 126)

Chapter 8: Port Trunking
Section I: Basic Operations 126
For example, when Workstation B sends a packet to the server, Switch #1
uses Port 15 of the trunk to transmit it to Switch #2.
An assignment of a source address to a port trunk remains active as long
as the source node remains active. If the MAC address times out, the
assignment is dropped. If the source node becomes active again and
needs to transmit a packet over the trunk, a new assignment is made,
either to the same port or to a different port in the trunk.
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 34 on page 125, 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. Since 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, of course, 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.