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We are unable to get the Ip address, I have checked DHCP server we have Ip address available, we can get the Ip address I connect windows system.

Please help me to resolve the issue... Thank you in advance...

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  • Are you saying that a windows system connected to the same network cable obtains an IP address ?
    – user9517
    Oct 15, 2010 at 8:21

3 Answers 3

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  1. Use packet sniffers. In Ubuntu(eth0 active interface)

    (first root console)# tcpdump -npi eth0 -s 0 -vvv

    (second root console)# dhclient eth0

  2. Read DHCP server log

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Other not-so-low-hanging-fruit: if this is a managed switch, check (or have the admin check) to see if spanning tree is enabled in classic mode.

I have seen Linux systems which are connected to such switches fail to pick up an IP address because they down and then up the interface before emitting a DHCP request; the switch interprets that as a physical disconnect/reconnect and initiates spanning tree on the link, and the problem is that even though the interface looks "up" to the linux computer, it isn't really, and very frequently DHCP emits its last request before spanning tree is finished with its dance.

Windows nodes don't seem to do the same thing with their interface when running the ifconfig/release/renew sequence.

Disable spanning tree entirely or enable in "rapid spanning tree (RST)" mode.

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  • I'm just perusing around to see if I can add value and since I'm not a CCNA or the like, spanning trees are a mystery I haven't tried to understand yet. This was an excellent addition and completely non-obvious solution for those of us used to blaming the operating system or its hardware. Thanks for thinking of it. Sounds like you lost some hours gaining this experience.
    – zerolagtime
    Oct 17, 2010 at 20:15
  • @zerolagtime you have no idea.
    – David Mackintosh
    Oct 18, 2010 at 6:12
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First check whether there is a problem with your network cable (try another cable). Next, set a static IP address and see if you can PING your DHCP server and that your static IP address is PINGable from elsewhere (this will also check the cable). If you manage to get cummunications working, re-set your computer to use DHCP and see what happens. If that doesn't resolve the problem, come back here.

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