Allied Telesis AT-S39 - Manual (Page 87)

AT-S39 User’s Guide
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For example, when Workstation B sends a packet to the server, Switch #1
will use Port 14 of the trunk to transmit it to Switch #2.
An assignment of a source MAC 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. Should the source node become active
again and need 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.
It should be noted that packets sent back from the destination node to
the original source node may travel the same or a different data link in
the trunk.
As a general rule, the SA 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 SA method be inappropriate? Returning to the
example in Figure 21, assume you configured Switch #2 also for SA 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, connected to Switch #2. Since
there is only one source, only one data link is used. So obviously the SA
method is not appropriate when there are fewer source nodes than data
links.
So now let’s look at the SA/DA method. A switch using the SA/DA
method creates a matrix of the source and destination MAC addresses
and then uses the matrix to determine which port in the trunk a frame is
to be transmitted. With this method, packets from a particular source
node might be sent over different data links in a trunk when sent to
different destination addresses.
So let’s take a look at how this might look in practice. Assume that you
configured Switch #2 in our example for SA/DA. The result might be
something similar to that shown in Table 2.
Table 2 Trunk Port Assignments in an SA/DA Matrix
Destinations MAC Addresses
Source MAC Address
Workstation
A
00A0EE 2313A3
Workstation
B
00A134 1A9032
Workstation
C
00A301 9083B2
Workstation
D
001B21 87C6D6
Server
00B012 DA0231 2 1 3 1