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New Member

DHCP problems in VLANs

I have a DHCP server setup in my default VLAN1. I have two other VLANs and I have configured iphelper address for those two vlans

I have no problem in the default vlan. I get the DHCP addresses fast without an issue. The dhcp server is also in default vlan

I have configured the ip helper address only in core switches. do I need to configure them for all individual switches. please let me know.

4 REPLIES

Re: DHCP problems in VLANs

You need to put the "ip helper-address x.x.x.x" under the Layer 3 interface where the client's default gateway is. for example, client in vlan x needs IP address from the DHCP server in vlan 1. At the layer 3 device, this would be the router or L3 switch doing the L3 or inter-vlan routing,under interface vlax x is where you will put "ip helper-address

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Re: DHCP problems in VLANs

Hi,

If your core switches are the default gateway for those users on those two VLANs and that's where you need to configure the 'ip helper-address (dhcp_ip)' command. If you have already done that and still having problems it might be due to spanning tree convergence. Configure

'spanning-tree portfast' on the ports to which users are connected.

Let us know if it helped.

Regards,

Sundar

New Member

Re: DHCP problems in VLANs

I had a similar problem and the only way I could get DHCP to work was to add the "Directed-Broadcasts" command to the vlan interfaces. Hope this helps.

Hall of Fame Super Silver

Re: DHCP problems in VLANs

Is it possible that your helper address configuration gave the address to forward to as a subnet broadcast (something like ip helper-address 172.16.6.255)?

This would require enabling directed broadcast on the interface where the server(s) is located.

HTH

Rick

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