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Cisco Swtich dhcp ip-helper address command

Last night I changed the IP address of my dhcp server which resides on a Windows server. I forgot to change the ip-helper entry for the vlan interfaces on our switch, so that broke dhcp. This morning I went in and removed the old ip-helper address entry and entered the new one. I thought that would take care of it but the requests aren't getting through. I can see by my dhcp logs that the dhcp server is not getting the requests. Is there something I have to do to clear/refresh so that the new ip-helper address entry works?

bsohn417

int vl xx
ip helper-address 172.16.x.x

bsohn417

u also have to remove old address by

int vlxx
no ip helper-address 172.16.10.10
ip ip helper-address 172.16.x.x

jpletcher1

ASKER

Yes, this is what I did this morning. I removed the old entry and added the new entry to the vlan interface, but it still doesn't seem to be working.

rochey2009

Hi,

From the PC, have you tried forcing a release then renew

ipconfig/release

ipconfig/renew

jpletcher1

ASKER

Yes, I have tried that many times and then also rebooted them.

jpletcher1

ASKER

This issue seems to be this... The new IP that I gave the dhcp server is on a different subnet/vlan than the clients. Before the clients were on the same subnet/vlan. I thought the ip-helper command was supposed to bascially relay all dhcp requests to the specified dhcp server, even if it is on a different subnet. Am I missing something?

rochey2009

can you ping the default gateway of the LAN where the PC's are located from the dhcp server?

Do you use DHCP snooping on your switches?

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jpletcher1

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rochey2009

That's correct, ip helper only works with the primary address.