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I would have a question regarding physical problem detection in a link with ping.

If we have a fiber or cable which has a problem and generate some CRC errors on the frame (visible with switch or router interface statistics), it's possible all ping pass because of the default small icmp packet size and statistically fewer possibilities of error. First, can you confirm this ?

Also, my second question, if I ping with a large size like 65000 bytes, one ping will generate approximately 65000 / 1500(mtu) = 43 frames, as ip framgents, then the statistics to get packet loss (because normally if one ip fragment is lost the entire ip packet is lost) with large ping is clearly higher ? Is this assumption is true ?

The global question is, with large ping, could we easier detect a physical problem on a link ?

bdes31
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  • Stack Overflow is a site for programming and development questions. This question appears to be off-topic because it is not about programming or development. See [What topics can I ask about here](http://stackoverflow.com/help/on-topic) in the Help Center. Perhaps [Super User](http://superuser.com/) or [Unix & Linux Stack Exchange](http://unix.stackexchange.com/) would be a better place to ask. Also see [Where do I post questions about Dev Ops?](http://meta.stackexchange.com/q/134306) – jww Jun 10 '17 at 05:30
  • Can you transfer my message to the networkengineering.stackexchange ? Thanks. – bdes31 Jun 10 '17 at 10:57
  • You can flag the question and ask the moderators move it so you don't have to delete it on one site and create it on another site. For whatever reasons, the Stack Exchange network does not allow you to migrate it yourself. Also see [Author initiated migration to other SE sites?](http://meta.stackoverflow.com/q/254851) – jww Jun 10 '17 at 10:58

1 Answers1

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A link problem is a layer 1 or 2 problem. ping is a layer 3 tool, if you use it for diagnosis you might get completely unexpected results. Port counters are much more precise in diagnosing link problems.

That said, it's quite possible that packet loss for small ping packets is low while real traffic is impacted more severely.

In addition to cable problems - that you'll need to repair - and a statistically random loss of packets there are also some configuration problems that can lead to CRC errors.

Most common in 10/100 Mbit networks is a duplex mismatch where one side uses half-duplex (HDX) transmission with CSMA/CD while the other one uses full-duplex (FDX) - once real data is transmitted, the HDX side will detect collisions, late collisions and possibly jabber while the FDX side will detect FCS errors. Throughput is very low, put ping with its low bandwidth usually works.

Duplex mismatches happen most often when one side is forced to full duplex, thus deactivating auto-negotiation and the other side defaults to half duplex.

Zac67
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  • Thanks for your reponse. I'm ok that ping is a layer 3 tool, but I think we could correlate ping with physical layer problem because of the layer 1 is used by upper layers. I'm looking for details regarding this correlation from a statistic point of view. I just saw it's in wrong section, I have to post my question to the networkengineering stackexchange section. Thanks. – bdes31 Jun 10 '17 at 11:05