all: We have just started using an existing Cat5e cable to run 100Base-T to a new office, but unfortunately, machines in the new office see link but receive an APIPA address (169.254.x.x).

I brought the same computer into the network closet and plugged it directly into the hub and everything works okay, so it must be the wiring through the walls.

Can I use Wireshark or some other (Mac) software to confirm this? Or is there an inexpensive hardware test, short of pulling the cable back through the walls?

Thank you, /m

  • 1
    Are you getting a link light at the ends of your longer cable runs? Is Wireshark seeing ARP traffic?
    – quux
    Sep 6 '11 at 9:34
  • Yes, link is lit, I just don't get an IP from DHCP. So I'm wondering if there's a way to determine with software. Will check if ARP is going back and forth.
    – mrisher
    Sep 8 '11 at 4:37

There are two main tests for Ethernet Cable:

  • Continuity Testing. This checks that all pairs are connected in the correct order and that there is a continuous signal from one end of the cable to the other. You can buy these testers for peanuts: http://en.wikipedia.org/wiki/Continuity_tester

  • Cat5e Cable Analysers. These testers cost upwards of AU$1k and they test the exact electrical conductivity, interference etc of the cable and let you know if it falls in line with Cat5 specs. Assuming the cable is good, and the jacks are fine (check with the continuity tester), this device will tell you if there is too much interference or signal loss which can happen when: The cable gets old, something wears/eats away the shielding, the cable is coiled tighter than regulations, the cable runs parallel to power cables etc. http://www.flukenetworks.com/datacom-cabling/copper-testing/dtx-cableanalyzer-series

You can hire these testers, but it can be expensive. It'll be cheaper than re-cabling your whole office.

Don't forget that there are Ethernet Specifications which dictate how long your runs can be, how many hubs/switches etc. AFAIK a hub means that both cables plugged into the hub need to be shorter than the Ethernet Max Length, where as a switch stores and forwards the packet, so perhaps you can get the max length on both sides of the switch. With the prices of switches, there's never any reason to use a hub.

  • Thanks for the clarification there; I was wondering the pricing of the Cable Analyzer, because I kept encountering only the continuity tester.
    – mrisher
    Sep 8 '11 at 4:35
  • 1
    Is there any way to emulate the Cable Analyzer functionality using software? Feels like if I'm getting link but not receiving a DHCP lease, there's something that's not quite making it through the pipe...
    – mrisher
    Sep 8 '11 at 4:36
  • In a manner of speaking. If you boot into BIOS, some computers have Ethernet Analysis options that will tell you the length of the cable and a few other items. You'll find the information is very limited and I always found these BIOS options often causes the computer to become unresponsive (in BIOS). There is no software that can do this because the requirements of analysing the physical cable are beyond the capabilities of an Ethernet PHY.
    – Dom
    Sep 8 '11 at 6:28
  • Is it happening on every Ethernet cable in the new office, or only one cable? If it's every cable, I'd suspect it a termination issue. Just run the $20 continuity tester over the cables to check them (or find an electrician friend who has one you can borrow).
    – Dom
    Sep 8 '11 at 6:30
  • Another thought - Are you using a Gigabit Switch and Gigabit capable computer? If so, certain wiring faults will cause Gigabit to claim it has a physical link, but be unable to transmit data. You can get around this issue by forcing the computer to Fast Ethernet. This only resolves ONE cabling termination oddity, and if the cables are terminated in any other incorrect way they still won't work.
    – Dom
    Sep 8 '11 at 6:47

You will probably get more "mileage" with a wiring test set to ensure there are no physical issues with the cabling (mis-match of pairs, length tolerance, physical damage, etc.).

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