Flow Control - Moog 903 User Manual

903-0623-00 Rev. A

5.5.3 Flow Control

The internal switching hub is a "learning switch" that monitors incoming packets to identify the MAC
addresses of the sources and establish a dynamic look-up table of up to 1024 addresses. Initially this
table is empty. Each time the switch receives a valid packet with a source MAC address not currently in
the look-up table, the source address is added to the table along with a time stamp. If the destination
MAC address is not in the look-up table, the packet is regenerated and forwarded to all other ports. Once
the destination MAC address is established in the look-up table, namely by receiving a packet from that
device, packets sent to that address will be regenerated only on the port identified in the look-up table.
Depending on traffic patterns, the switch typically learns all of the MAC addresses within a few seconds,
though it may take longer to identify stations that do not generate much traffic. During the learning period,
many packets will be copied to all ports.
If no packets are received from a given MAC address for 375 seconds, the aging feature of the switch
removes the look-up table entry. If an external device is moved from one port to another on the same
EIB-10/100 card, the migration feature of the switch immediately updates the table as soon as a new
packet is received. If, however, an external device is moved from the switch at one end of the system to
the switch at the other end, it may take up to the aging time (375 seconds) before the link through the
multiplexer is properly re-established. Consider this behaviour when switching cables around during
bench tests.
The switch will not forward the following types of packets:

Error packets due to framing errors, FCS errors, alignment errors, and illegal size frames

802.3x pause frames

"Local" packets. Packets are deemed "local" if the destination address from the look-up table
matches the port of the incoming packet, e.g. from old repeating hubs

Broadcast or multicast packets that exceed the threshold when broadcast protection is enabled
Focal Technologies Corp.
Model 903 User's Guide, FMB-X-2.5 Version
Page 5-21