ip helper address

[jjames]

I would like to add a secondary site for DHCP support. Currently all of my remote sites come to one site for ip address assigments. I would like to add a secondary address for DHCP should the primary fail. Can I do this? How?

[Richard Burts · Accepted solution]

I am assuming that you have configured the remote sites with the command ip helper-address which points to the address of your primary DHCP server. And now you want to add another DHCP server.

You can certainly add a second address (for another DHCP server). However your message seems to indicate that you want it to function only when the primary fails. It will not work this way from Cisco routers. If you add a second address (for a second server) when an end station sends a DHCP request, the router will receive it and will forward it to BOTH of the configured addresses. So both servers will see the request, and probably both will answer. If both servers do answer then the end station will choose the one it decides to use. The other DHCP server will realize that its offered address is not being used and will withdraw the address and make it available for other requests. This is actually normal behavior for DHCP and should not be a problem.

To add the second server you will just configure the remote routers with another ip helper-address command which will have the second server's address.

HTH

Rick

[Aaron Harrison]

You'll also want to make sure that your DHCP servers are configured correctly - i.e that they aren't both giving out the same addresses.

The normal way to do this (for Windows at least) is to set up the same scopes on both servers, but put a range of exlusions that covers half on one, and the opposite half on the other server...

If you don't do this you end up with servers sending NAKs in response to legitimate renewal requests and so on which can cause pain and waste (a little) bandwidth..

Aaron

[jjames]

Thanks that answered my question