Helper-address

[maryodriscoll]

Hi,

I want a bunch of users to get an IP address from a DHCP server located in Head Office. There are at least 2 wan hops from the spoke site to Head Office and from there multiple different vlans before the DHCP request would reach the DHCP server.

I understood that all I need to do is enable ip helper-address on the requesting source IP ethernet interface of the router but while I can see the outgoing bootp request, there is no reply coming back.

All cisco routers are running IOS > 12.0. Am I missing anything?

DHCP server is up and I have confirmed IP connectivity from the SRC ip address by doing an extended ping.

Thanks

Mary

[Richard Burts]

Mary

In terms of router configuration the most important thing is to configure ip helper-address on the Ethernet interface where the DHCP request will be received from the clients. It should not be necessary to configure anything on any intermediate router. If you would happen to configure the ip helper-address as a directed broadcast (the address specified is the broadcast address of the segment where the server(s) are located), you would need to be sure that ip directed-braodcast is enabled on the router interface where the server is.

There has been some discussion indicating that ip helper-address might need service dhcp to be enabled. You might check that and enable it if it is not enabled.

The other thing that I would suggest is to verify that the DHCP server is configured with a correct scope for the remote subnet where these clients are.

HTH

Rick

[maryodriscoll]

Thanks Rick, This helps. I have configured ip helper-address X.X.X.X where x.x.x.x is the specific host IP address of the DHCP server. Service DHCP is enabled but I have kept no ip directed-broadcast on the ethernet and wan interface. This was the bit I was a bit concerned about.

[Richard Burts]

Mary

If the helper address specifies the unicast address of the DHCP server, then directed broadcast does not come into play.

And if it did come into play the only place you need to have it is on the outbound interface to the segment where the server is located. (You do not need to specify directed-broadcast on any intermediate router).

If it is not working I would look first at the DHCP server. Does it have a correct scope defined for these clients? Are there logs on the server that indicate whether it saw the request arrive? Are there logs that indicate that it attempted to offer an IP address?

There are some debugs that you could run on the router that might also provide some insight.

HTH

Rick