01-05-2007 01:58 PM - edited 03-05-2019 01:37 PM
Hi all, if I have a switch with 3 vlans, and 1 windows dhcp server, Do I need to issue the command once, or on each vlan ?
01-05-2007 02:00 PM
Each vlan interface that will receive dhcp requests will need the ip helper command.
01-05-2007 03:24 PM
any subnet that is not directly attached to the dhcp server subnet will need the helper statement .
01-05-2007 04:55 PM
You'd need an IP Helper on each router / L3 interface that is not a member of the VLAN on which the DHCP server resides.
You don't mention it specifically, but you will need a router or L3 switch to get the VLANs to talk to each other.
The IP Helper goes on the client-side of the router and points to the DHCP server.
Good Luck
Scott
01-05-2007 05:11 PM
If you do enable the ip helper-address on each VLAN remember that it impliciitly enables fowarding of several different kinds of UDP broadcasts.
udp tftp
udp nameserver
udp domain
udp time
etc
this may not be desirable
Mark
01-06-2007 04:48 AM
Basically, "ip helper" command forwards broadcast as unicasts to the specified address. The command is issued for every Interface VLAN (broadcast domain) from which you want to forward broadcasts to some destinations (e.g. DHCP server, TFTP server, and so on)
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