DHCP ip helper-address questions

[bfanion]

I have a pair of 6509's with many vlans that I have configured a pair of DHCP servers via the ip helper-address command.

One of the servers is local (attached to core switch) The other server not so local (across a T3 line).

Some of the clients are getting a lease off of the 2nd non-local DHCP server. Is there a way to set a prefered order for this, I assumed that the order in the config was the factor. I would rather not have clients pulling a lease from the remote server (unless of course the local server was not operational).

Thanks in Advance

[Richard Burts]

Barry

The order of addresses in the config has nothing to do with which DHCP server the client will get its address lease from. And there is very little that you can do to control this. The way it works is as follows:

- client sends a DHCP request which is a broadcast.

- 6509 with two helper addresses configured will forward the DHCP request to both servers.

- each server will respond with DHCP offer.

- client will choose which offer to use (typically chooses the first response it receives).

- if for some reason the "local" server is a bit slow to send its response (queued up with other tasks or whatever) then it is likely that the client will accept the offer from the server over the T3.

- not much you can do to change this.

I also wonder if it matters much which server the client gets the address from. The DHCP request is going to be sent to both servers no matter what, both servers will send a response, the packet from the client indicating the offer that it accepts is sent to both servers. So you are not really reducing traffic. So does it matter which server is being used?

HTH

Rick