FCM9002 USER MANUAL
76-02-095 Rev.B
Page 131 of 145
4.7.2.2 SAM Loopback
The FCM9002 has a built in wirespeed loopback facility which will perform MAC
source and destination address swapping. The loopback operates at full
wirespeed, and supports frames of size up to the maximum MTU setting for the
device. ( 1522/2048 or 10k )
For ease of use the Loopback uses “magic” MAC addresses as the destination
address for test frames. Any frame received with a “magic” destination address
will be switched to the loopback module and returned to the port it was received
on but with source and destination addresses swapped. The remainder of the
packet will be unchanged, however the FCS will be recalculated if necessary.
The magic destination MAC addresses are as follow:
4.7.2.2.1 WAN Port
Any frame ingressing the selected WAN port with a “magic” address will be
looped back regardless of whether a flow has been configured or not.
There are two “magic” addresses supported by the FCM9002 WAN port
4.7.2.2.2 Well Known “Magic” Address
00:C0:81:FF:00:00
This is common to all Metrodata MetroConnect
devices that support DA/SA swap loopback
and as such may be used as the destination
address for loopback from any unit.
4.7.2.2.3 Device Specific Loopback Address
In some networks, the use of the common, well known target address may cause
problems due to the same address seeming to reside in many different locations
due to the DA/SA swapping function. If this is the case, then a device specific
loopback address may be used, however this requires knowledge of the MAC
address of the target unit. Once the target MAC address is discovered, then the
“magic” address is generated by setting bit 23 of the MAC. The example below
shows this in practice.
