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New Member

DHCP Over WAN

question -

i have a dhcp server on the other side of my WAN. on both ethernet interfaces (one on this side and the interface the dhcp server is on)I have the helper address configured. I can't seem to obtain a dhcp address. the scopes have been defined. do i need to add the helper address to the serial links? what if the "no ip directed broadcast" is stated on the interface?

9 REPLIES
New Member

Re: DHCP Over WAN

If your DHCP server's address is 10.1.1.1/24 then you should have an IP HELPER 10.1.1.1 statement on the client's segment ethernet interface of your router.

You can also use IP HELPER 10.1.1.255 to send a directed broadcast to the 10.1.1.0/24 segment. If you do this, you will need to enable IP DIRECTED-BROADCAST on the server segment router's ethernet interface.

IP HELPER was also broken in some releases of 12.0T... check your version. I know it was broken in 12.0(4)T and 12.0(5)T... maybe even 12.0(7)T depending on your config.

Mick.

New Member

Re: DHCP Over WAN

so, if my server address is 10.1.1.26 then the helper address should be 10.1.1.1? I have been setting the helper address as the address of the server...

New Member

Re: DHCP Over WAN

The IP should be the one of the server. This should be set on the the ethernet interface on the end remote from the DHCP server and not on both ends. I have also noticed that it may help to turn on IP-DIRECTED BROADCASTS on some dificult routers, and use the "ip forward-protocol udp bootpc" command to make sure that the bootp requests are forwarded. Another factor is if you have secondary addresses on the Ethernet interface with the IP helper, the IP helper only forwards from the primmary address. IF you are using NT DHCP servers the scope for that subnet needs to be set and you should be able to recieve addresses on the remote end because of RFC 1542 complience. The server needs to be RFC 1542 complient. The forward process will add the router interface to the request and forward it to the server. The server will look at this address and see the remote routers IP address and the server will respond with the appropriate address for that subnet. The scope on the DHCP must match the subnet on the remote gateway address. Hope this helps. You may also want to goto the NPC home page and do a search on DHCP Relay. This will give you a lot of information on the topic.

New Member

Re: DHCP Over WAN

you are correct, there are secondary interfaces on the routers interfaces. why would this work on the LAN but not over the WAN. do i need to add this command to the serial interfaces?

New Member

Re: DHCP Over WAN

also, what if I have the helper address on both interfaces? which side do I add the bootp upd command?

thnks for the help

New Member

Re: DHCP Over WAN

On the Ethernet interface for the lan is where all this will be added, The ip helper the ip directed and the bootp commands. The IP helper local to the DHCP will cause extra traffic due to the workstation and the router doing requests for each workstation DHCP enabled. The following is an example:

interface Ethernet0

ip address 10.58.96.1 255.255.224.0 secondary

ip address 10.13.0.1 255.255.0.0

ip helper-address 10.6.1.10

ip directed-broadcast

no ip route-cache

This is the remote end. and the DHCP is on the 10.6.0.0 subnet with the address of 10.6.1.10. We are serveing to the 10.13.0.0 subnet and the secondary is for go live testing for a system refresh that we are doing and have to change the addresses. so this will work on the primary address and not the secondary. The BOOTP UDP command is to just verify that the bootp requests are forwarded and should be used on each end. We have the routing for our remote office hop across 2 routers befor hitting the DHCP. Do you need both addresses served from DHCP?

James

New Member

Re: DHCP Over WAN

One more question is the secondary you mentioned a subinterface or a secondary address?

New Member

Re: DHCP Over WAN

the net 10 is the secondary. here's the config at the remote office:

description Main subnet

ip address 10.x.122.1 255.255.254.0 secondary

ip address x.x.180.1 255.255.255.128

ip helper-address 10.x.0.26

speed 100

full-duplex

!

Does this ok? This should work, right?

New Member

Re: DHCP Over WAN

ip address x.x.180.1 255.255.255.128 Is the net served the DHCP in this case. Yes this should work for this subnet only if the scope on the dhcp are correct for that subnet. The only subent covered by DHCP would be ip address x.x.180.1 255.255.255.128

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