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New Member

DHCP across WAN

hi all,

In a few months we will be deploying a new design with very centralized servers. in our remote sites they will have domain controller to reach for dhcp, but in the even that it should die, i would like to able to assign an ip over the wan from the home office. what router considerations do i need to have to allow the dhcp request to reach the server? should i be using ip helper addresses? I will already be doing this somewhat as my dhcp server will reside on a different vlan that my hosts.

TIA,

R

5 REPLIES
New Member

Re: DHCP across WAN

I haven't done NT in a long time, but I beleive you can use NT to setup a backup DHCP server/scope. As for forwarding that traffic you will need to use helper addresses for the primary configuration.

New Member

Re: DHCP across WAN

Yes, yu need to use the ip helper-address command. Also, if you have any access list or firewalls in between the sites you need to ensure the traffic is allowed. I am assuming you already have the correct routing in place as well.

New Member

Re: DHCP across WAN

Thanks for the replies. I know the helper command goes on the vlan interfaces inside the lan, but in the wan, i was unsure. would this command go on the interface facing the switch, be it fast ethernet or what have you. or on the int facing the wan?

also, i have seen posts with the following to lock down the helper command:

o ip forward-protocol udp tftp

no ip forward-protocol udp dns

no ip forward-protocol udp time

no ip forward-protocol udp netbios-ns

no ip forward-protocol udp netbios-dgm

no ip forward-protocol udp tacacs

Are these needed?

Re: DHCP across WAN

The ip helper-address command is always applied on the LAN interface. It doesn't serve any purpose to configure it on the interface facing the WAN side unless the router interface on the other end is configured as a DHCP client.

When helper-address is configured the router listens for broadcast on the wire and takes it and converts to directed broadcast/unicast packets depending on what the helper address is pointing to.

HTH,

Sundar

New Member

Re: DHCP across WAN

When you enable the ip helper-address command on an interface it turns on udp forwarding for ports 69, 53, 37, 137, 138, 67, 68, 49,and 42. If you want to prevent other protocols from being forwarded you should use the no ip forward-protocol command.

If this helps please rate my post, thanks.

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