Using 1231 with Cable Modem - Passing through DHCP

Unanswered Question
Oct 18th, 2009

Hey all,

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.

interface BVI1

ip address dhcp client-id FastEthernet0

no ip route-cache

interface Dot11Radio0

no ip address

no ip route-cache


encryption mode ciphers aes-ccm


ssid vikings


speed basic-1.0 basic-2.0 basic-5.5 6.0 9.0 basic-11.0 12.0 18.0 24.0 36.0 48.0


station-role root

bridge-group 1

bridge-group 1 subscriber-loop-control

bridge-group 1 block-unknown-source

no bridge-group 1 source-learning

no bridge-group 1 unicast-flooding

bridge-group 1 spanning-disabled

interface FastEthernet0

no ip address

no ip route-cache

duplex auto

speed auto

bridge-group 1

no bridge-group 1 source-learning

bridge-group 1 spanning-disabled

The AP gets an address, but no clients can get addresses once associated to the AP

I'm hoping it's just a config issue, and not the fact that my ISP is limiting my addresses, but I'm afriad that might be the case.

I just want to confirm here that I have this setup correctly.

Thanks again!

I have this problem too.
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richardhartman Mon, 11/02/2009 - 13:46

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!


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