IP helper address stops working

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Jan 21st, 2009
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Does anyone have any suggestions for debugging a problem with the ip helper address?

Twice in the last 24 hours, a C3560E, Version 12.2(44r)SE2 has stopped forwarding DHCP requests on a particular VLAN. In both situations, forwarding to the same DHCP server did continue to work fine from another VLAN.

The fix for the problem was to remove the ip helper address line from the config, wait 30 seconds and put exactly the same line back in again! What was also odd was if we put in a second helper, that got the DHCP requests without a problem while the original remained silent.

I have tried a 'debug ip dhcp server packets' and that suggests it is trying to forward to the correct address. Debugging output looks the same for both a good or bad period.

I'd appreciate any other debugging suggestions anyone has.

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Yudong Wu Wed, 01/21/2009 - 12:37
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packet sniffer will be helpful here.

You can capture on this 3560E or a device between this one and DHCP server when the issue happens.

You can also ask server guys to check DHCP server to see if IP Pool for that VLAN is full.

a.tolstykh Thu, 01/22/2009 - 10:09
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Most likely the DHCP scope for this VLAN is out of the available addresses

davehartburn Fri, 01/23/2009 - 00:58
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Thanks for the responses.

Sniffing from the DHCP server confirms requests are not passed through. If we were out of addresses, that would be reported in the log.

Yudong Wu Fri, 01/23/2009 - 07:38
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Hi Dave,

Can you do a sniffing hop by hop to see if you could find where the DHCP packet is dropped?

Please let me know the software version so that I can look up for a bug.

By the way, "ip helper" configured under the other vlan interface on the same switch works OK. is that right?

mlitka Mon, 01/26/2009 - 10:04
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Are there secondaries configured on this interface?


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