When dialing up to a a Cisco BRI port (with the IP helper address configured) I get an error stating that a compatible set of protocols could not be negotiated. Dial-up is fine if I configure a static IP for the client, but I was expecting DHCP to work. I have configured a DHCP scope in the same way as usual, and this works for modem dial-up, but for ISDN I am having to resort to static addresses. Can anyone tell me what I've missed?