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How does the "interface detection link" mechanism work?


The "lost carrier" counter is periodically incrementing on a gigabit interface (sh int) connected to a service providers equipment. Documentation indicates this counter increments based on the "interface detection link" mechanism, but I can find no useful information on how this works.

0 interface resets

0 babbles, 0 late collision, 0 deferred

4 lost carrier, 0 no carrier, 0 pause output

4 Replies 4


I'm not sure that you did check this information already or not.

lost carrier::

Description: Cisco IOS sh interfaces counter. The number of times the carrier was lost in transmission.

Common Causes: Check for a bad cable. Check the physical connection on both sides.

Here you go:



Thanks Toshi,

I do have that link which is very useful in narrowing the items to check. But I'm looking for an explanation of how the interface detection link mechanism actually works. The knowlegede would be useful in working out the fix with our provider.



The mechanism is at layer 1, so the exact details depend on the physical layer of your adapter and the type of cable you use. You said this is a gigabit interface, but it could be over UTP or optical. The details can be found in the IEEE 802.3 (but takes some time to download, so I don't have exact documentation to provide to you right now). The general idea for practical purposes is typically the following: Each side of the link has some transmit wire(s) and receive wire(s). The receiver at each side can detect if the remote side is sending a signal. So, if your local receiver reports lost carrier there are chances of problems with remote transmitter or intermediate network in the transmit path towards your receiver. Local side usually cannot see problems with the signal it transmits, unless the remote side complains about it. This means that the remote side could see interface up because your side sends good signal, while your side sees interface down because they send bad signal.

OK, I managed to download the standard. Here are two relevant samples from it for optical media:

" PMD_SIGNAL.indication

This primitive is generated by the PMD to indicate the status of the signal being received from the MDI.

The semantics of the service primitive are PMD_SIGNAL.indication(SIGNAL_DETECT). The SIGNAL_DETECT parameter can take on one of two values: OK or FAIL, indicating whether the PMD is detecting light at the receiver (OK) or not (FAIL). When SIGNAL_DETECT = FAIL, PMD_UNITDATA.indication(rx_bit) is undefined. The PMD generates this primitive to indicate a change in the value of SIGNAL_DETECT.

SIGNAL_DETECT = OK does not guarantee that PMD_UNITDATA.indication(rx_bit) is known good. It is possible for a poor quality link to provide sufficient light for a SIGNAL_DETECT = OK indication and still not meet the specified bit error ratio."

"59.2.4 PMD signal detect function

The PMD signal detect function shall report to the PMD service interface using the message

PMD_SIGNAL.indication(SIGNAL DETECT) which is signaled continuously. PMD_SIGNAL.indication

is intended to be an indicator of optical signal presence.

The value of the SIGNAL_DETECT parameter shall be generated according to the conditions defined in Table 59-2. The PMD receiver is not required to verify whether a compliant 1000BASE-X signal is being received. This standard imposes no response time requirements on the generation of the SIGNAL_DETECT parameter.

As an unavoidable consequence of the requirements for the setting of the SIGNAL_DETECT parameter, implementations must provide adequate margin between the input optical power level at which the

SIGNAL_DETECT parameter is set to OK, and the inherent noise level of the PMD due to cross talk, power supply noise, etc."

The standard can be very formal in its expressions, but the idea is the following: The receiver checks input optical power levels to detect reception of good signal from the remote transmitter.

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