Using 1231 with Cable Modem - Passing through DHCP
I've been trying to configure my AP for a couple of hours now without any luck.
I want my AP to hand out IPs directly from my ISP. I've done this before, and I remember it being very simple. I setup the BVI to pull DHCP addresses, and configured the rest of the AP for authentication, etc., and it just worked.
Now I can't get the AP to hand out any IP addresses.
Here is my BVI config, along with the relevant radio and fa0 configs.
Re: Using 1231 with Cable Modem - Passing through DHCP
I recently worked on a cable-based "broadband" connection locally and found this behavior there, too. Numerous experiments with Cisco and non-Cisco equipment seems to confirm that they will issue exactly one IP via DHCP.
Furthermore, it seems that the issued IP is tied to the MAC address of the device that is physically connected to the cable modem. Example: If you assign a fixed IP to your AP/bridge's BVI and then wirelessly associate a laptop that wants a DHCP address, that laptop cannot obtain the one-and-only IP address from the cable ISP. You would think that the bridge would just transparently move the DHCP packets back and forth, letting the laptop act as the "only" device. But it won't.
If connected directly to the cable modem, the laptop gets a DHCP address just fine. And if the AP/bridge is set up to itself use DHCP, it too can get an address when connected directly to the cable modem. But nothing downstream of the directly connected device can obtain the DHCP address.
At first I thought this behavior was caused by the AP/bridge not bridging DHCP packets (which are UDP). But I've since confirmed that 1) the bridge is passing such traffic, and 2) other non-Cisco devices experience the same problem.
Finally, if you wish to use a different "single" device (i.e. a different laptop, etc.) the cable modem must be power-cycled.
I don't know if this single-DHCP behavior is part of the DOCSIS spec, or something that individual cable companies choose to do on their own. We've even tried two different cable modems, by different manufacturers, complying with different versions of the DOCSIS spec, and the behavior is consistent.
For this site, the bottom line is that only one DHCP address is available, and that address must go to the device connected directly to the cable modem. Plan accordingly!