Comtech EF Data CDM-750 - Manual (Page 97)

CDM-750 Advanced High-Speed Trunking Modem Revision 2
Front Panel Operation MN-CDM750
5–19
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 set as 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 is “learned” to exist as previously described. This learned
address will remain learned, and it will continue to exist in the modem’s Content
Addressable Memory (CAM) table for a period of five (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 will remain in the table and the aging clock will reset once more to five
minutes.
It is important to note that, by enabling MAC Learning, the amount of traffic over
the satellite link can be reduced when the modem is one element on the LAN (see
Figure 5‐2, Detail ‘A’). However, if there is the chance that a second Layer 2 path
exists to the far side equipment, this will cause data flow failure. This condition is
known as “Spanning Tree” (see Figure 5‐2, Detail ‘B’). This means that the MAC
Address of a device – such as PC1 in the figure – appears to exist on both the
WAN and LAN sides of the modem. This corrupts the modem’s CAM table and
causes its failure.
Figure 5-2. MAC Learning Operations