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DHCP problems

Hi folks

We have discovered an evolving problem. Our customers - normally small sites with 5 - 25 PC's get their IP config from an DHCP server in an Cisco SOHO router - typically an 831 with IOS 12.2(8) or similary. We also often have an Cisco Catalyst switch (CAT 2950) in there. The clients - Windows 9x, Windows 2kPro & XP Pro from time to time boots without an DHCP assigned IP - hereby booting with an 169.x.y.z address and those problems which this can initiate for a normal PC User!.

Is this an know issue when using the DHCP service in these SOHO routers - or - help - what can be done - any suggestions?? I expect the cause for these problems are either dropped packets so that the Cisco router newer receives an DHCP request - or it is to slow to process them (hmm - for 5 - 25 clients - doesnt make sense) Any suggestions for tracing - debugging or alternate configs??

Kind Regards

Thomas Iwang


Re: DHCP problems

Hi Thomas,

this may be a timing problem.

When a link on a switch becomes active it will generally take around 40 seconds before the switch actually passes traffic over the port due to the spanning tree protocol, so it is possible that the PC has already sent its DHCP request when the switchport was not yet active.

For PC's directly connected to the switch, you can configure their switchport with "spanning-tree portfast". This shortens the time between the different spanning tree stages, so never do this on port connected to a hub or another switch.



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