Newly provisioned circuits are often put into loopback with a physical loopback cable for testing and to ensure the ports meet the SLA. If loopbacks are not cleared, or physically removed, by the operator when the testing is completed, they can adversely affect the performance of all other SDPs and customer interfaces (SAPs). This is especially problematic for point-to-multipoint services such as VPLS, since Ethernet does not support TTL, which is essential in terminating loops.
The down-when-looped feature is used on the 7705 SAR to detect loops within the network and to ensure continued operation of other ports. When the down-when-looped feature is activated, a keepalive loop PDU is transmitted periodically toward the network. The Ethernet port then listens for returning keepalive loop PDUs. In unicast mode, a loop is detected if any of the received PDUs have an Ethertype value of 9000, which indicates a loopback (Configuration Test Protocol), and the source (SRC) and destination (DST) MAC addresses are identical to the MAC address of the Ethernet port. In broadcast mode, a loop is detected if any of the received PDUs have an Ethertype value of 9000 and the SRC MAC address matches the MAC address of the Ethernet port and the DST MAC address matches the broadcast MAC address. When a loop is detected, the Ethernet port is immediately brought down.
Ethernet port-layer line loopbacks and the down-when-looped feature can be enabled on the same port. The keepalive loop PDU is still transmitted; however, if the port receives its own keepalive loop PDU, the keepalive PDU is extracted and processed to avoid infinite looping.
Ethernet port-layer internal loopbacks and the down-when-looped feature can also be enabled on the same port. When the keepalive PDU is internally looped back, it is extracted and processed as usual. If the SRC MAC address matches the port MAC address, the port is disabled due to detection of a loop. If the SRC MAC address is a broadcast MAC address because the swap-src-dst-mac option in the loopback command is enabled, then there is no change to port status and it remains operationally up.
EFM OAM and down-when-looped cannot be enabled on the same port.