Our team recently setup a new network and now we are experiencing network latency issues. the flow of traffic is as such: modem -> router -> packet shaper -> cisco3750 switch

from router to packet shaper, I believe we used a cross over cable and from packet shaper to switch, a straight through.

My question is, would using a crossover cable vs straight through cause noticeable lag in the network? The client site is a few hours away and am hoping this is not the case :(

UPDATE: Here are some speed test results

The latency is very noticible during VOIP calls, we have 2 sites and a T1 connecting the two. The above speed test results are from one of the sites.

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    @KPS, if the devices support it, can you provide any equivalent show interface <interface> output for the router interface, the switch interface and the shaping device's interfaces? Being able to see this information should allow someone to provide an accurate answer and not just a guess. – YLearn Jun 18 '14 at 20:17
  • @YLearn, I am trying to obtain that info as we speak. I will update above once I have. – KPS Jun 18 '14 at 20:22
  • Latency is not the same as throughput. Although a highly loaded link can affect the latency of other connections if traffic shaping isn't set up right. And traffic shaping is very hard to get right. – Matt Jun 18 '14 at 20:31
  • An old cable will cause this problem as it won't meet the newer standards for fast or gigabit Ethernet. If somebody dug up an old crossover cable thinking it was needed between two devices then thrown it out. Don't need a cross over cable, do need a proper CAT5e or CAT6 cable properly crimped. – Brian Jun 18 '14 at 20:49
  • Unless this is for a 10 Mbit circuit. We don't know enough to say for sure. – mfinni Jun 19 '14 at 1:09

In your question's scenario : No. It would either work (meaning that your ports have Auto MDI-X, which is built into Gigabit Ethernet, IIRC) just fine, or not work at all.

Which means the cause of your problem is elsewhere.

  • I don't believe this answer is correct. The vast majority of crossover cables don't work with gigabit Ethernet. I suspect this is because most crossover cables predate gigabit Ethernet and so don't comply with the gigabit crossover standard (which requires all pairs be swapped). – David Schwartz Jun 18 '14 at 19:16
  • In which case, it would not work at all, as I said in my answer. I don't think you have a failure mode, related to the use of a crossover cable, that results in added lag. – mfinni Jun 18 '14 at 19:39
  • Whether or not they won't work at all depends on the configuration. I think you're right that with a default configuration at both ends, it probably won't work at all (at least on most devices). But I wouldn't assume that the defaults haven't been changed. – David Schwartz Jun 18 '14 at 19:45
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    If it's a brand new crossover cable that was intended to be used with a gigabit device, it's unlikely to be the problem. (Though I can't understand why anything would ship with such a cable. Modern devices universally have auto-MDI/X and don't need it.) – David Schwartz Jun 18 '14 at 20:19
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    @DavidSchwartz - sure, a bad cable could cause some problems. The specific question was if a crossover cable would cause the described problems. My answer is still "no." – mfinni Jun 18 '14 at 20:19

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