METRODATA NetTESTER FAMILY USER MANUAL
76-02-107 Rev.A
Page 45 of 133
00:C0:81:4x.xx.xx*
Any User
* - x.xx.xx is the remainder of the unit MAC address, i.e. this is the unit MAC address modified by
setting either bit 23 or 22.
** - These are referred to in places as "Magic MAC" addresses.
When a packet is looped, the source and destination MAC addresses will be
swapped, and the FCS recalculated.
10.2.2
Layer 3 Loopback
In Layer 3 loopback mode, the unit will perform the Layer 2 loopback, as above. In
addition, it will loop packets where:
•
The destination MAC address matches the (unmodified) address of the unit,
and
•
The packet carries an IP payload, and
•
The IP protocol number is equal to a selectable value (the default value is
254), and
•
The packet is received on the WAN port.
All looped packets will have their source/destination MAC addresses swapped and
FCS recalculated, as per the Layer 2 loopback.
In addition, packets carrying an IP payload will have their source/destination IP
addresses swapped, the TTL field reset to a value of 32 and the IP checksum
recalculated.
The Layer 3 loopback mode is intended for use with third-party Ethernet testers such
as the JDSU Smartclass or Sunrise MTT which do not fully implement the IP packet,
simply placing a protocol value in the IP header, but not actually defining the protocol
payload.
10.2.3
Layer 4 Loopback
In Layer 4 loopback mode, the unit will perform the Layer 2 loopback, as above. In
addition, it will loop packets where:
•
The destination MAC address matches the (unmodified) address of the unit,
and
•
The packet carries an IP and UDP payload, and
•
The UDP port number is equal to a selectable value (the default value is 7),
and
•
The packet is received on the WAN port.
All looped packets will have their source/destination MAC addresses swapped and
FCS recalculated, as per the Layer 2 loopback.
