Comtech EF Data CDM-760 - Manual (Page 117)

Front Panel Operation Revision 2
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
6–31
• With MAC Learning Disabled, the modem passes any traffic Entering from the GBEI (LAN)
interface to the satellite (WAN) side of the link, while traffic coming in from the satellite
(WAN) side of the link is passed on to the GBEI (LAN) interface. There is no filtering of traffic,
and the modem connection looks like a “wire.”
• With MAC Learning Enabled, the Destination MAC and Source MAC are “learned” by the
modem. If the modem sees a destination MAC on its LAN side that it recognizes as belonging
to the LAN side, it will not transmit the frame. If the modem sees a destination MAC on its
WAN side that it recognizes as belonging to the WAN side, it will not transmit it to the LAN
side. If the modem sees a Source MAC on its LAN side, it learns that going forward. Any
Destination MAC it does NOT know, it will send across to the other side.
The MAC Learning process is further explained:
Frame going from LAN to WAN (Tx):
Source MAC Destination MAC Action
Don’t care Unknown Packet sent to WAN, Source MAC is learned to exist on LAN
Don’t care Known to exist on LAN side Packet NOT sent to WAN, Source MAC is learned to exist on LAN
Don’t care Known to exist on WAN side Packet is sent to WAN, Source MAC is learned to exist on LAN
Don’t care Broadcast or Multicast MAC Packet is sent to WAN, Source MAC is learned to exist on LAN
Frame going from WAN to LAN (Rx):
Source MAC Destination MAC Action
Don’t care Unknown Packet is sent to LAN, Source MAC is learned to exist on WAN
Don’t care Known to exist on LAN side Packet is sent to LAN, Source MAC is learned to exist on WAN
Don’t care Known to exist on WAN side Packet is NOT sent to LAN, Source MAC is learned to exist on
WAN
Don’t care Broadcast or Multicast MAC Packet is sent to LAN, Source MAC is learned to exist on WAN
The second process that is performed, with MAC Learning Enabled, is “aging”. This is why MAC
Leaning is also referred to as “learning and aging”. When a MAC Address is seen by the modem,
it is “learned” to exist as described previously. This learned address remains learned and
continues to exist in the modem’s Content Addressable Memory (CAM) table for 5 minutes. If
the MAC Address is not seen traversing through the product during this five minute period, the
table entry is “aged” out, and the MAC Address is no longer known to the modem and must be
re-learned. If, however, the MAC Address is seen within the CAM table’s five minute timer, the
MAC Address remains in the table and the Aging Clock resets once more to five minutes.