EXFO ETS-1000 - Manual (Page 78)

Configuring and Starting a Test
70 ETS-1000
Loopback
Loopback
The Loopback function is necessary for networks testing in compliance
with the RFC 2544, as well as for a number of other tasks. This function
allows to test the network without changing its settings. Network testing
with the Loopback function can be performed at the following OSI layers.

At the Physical layer (L1) all the incoming traffic is re-transmitted
backward without change; statistics are gathered for the incoming
traffic.

At the Data link layer (L2), all the incoming frames are re-transmitted
backward with swapping destination and source MAC addresses or
without swapping MAC addresses. ETS-1000 supports substitution of
destination and/or source MAC address with user-defined MAC
address. Both incoming and outgoing traffic statistics are gathered.
Note: For the loopback at the L2 or L3 layers, packets with identical destination
and source MAC addresses as well as OAM protocol data units (OAMPDU)
and ARP-requests, are not re-transmitted.

At the Network layer (L3) all incoming packets are re-transmitted
backward with source and destination IP addresses swapping.
ETS-1000 supports substitution of destination and/or source IP address
with user-defined IP address. Both incoming and outgoing traffic
statistics are gathered.
Note: if the incoming packet contains the label, it will be retransmitted without
changing it’s value

At the Transport layer (L4) all incoming packets are being
retransmitted backward (without error packets) with source and
destination IP addresses and source and destination TCP/UDP port
numbers swapping.
Note: if the incoming packet contains the label, it will be retransmitted without
changing it’s value.