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IP helper address stops working

Level 1
Level 1

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.

5 Replies 5

Yudong Wu
Level 7
Level 7

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.