[Resolved] Weird Half-Bridging / DHCP Issue

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Sambo
DD-WRT Novice


Joined: 18 Aug 2007
Posts: 27

PostPosted: Sun Nov 18, 2007 6:39    Post subject: [Resolved] Weird Half-Bridging / DHCP Issue Reply with quote
I have the following setup:

Linksys AM300 ADSL Modem in Half-Bridge Mode
Linksys WRT54GL with DD-WRT v23 SP2 installed obtaining the external IP from the ADSL modem
Standard ADSL Connection with dynamic IP

Previous to this week, this setup ran flawlessly until a couple of days ago it stopped working properly.

What is happening is that the modem is connecting fine & picking up the assigned Internet IP, Subnet and Gateway addresses as normal and connectivity from the modem works fine. Also, the DHCP on the modem works (to a point) and the router picks up an IP address, subnet and gateway from the modem. However, while the modem is getting a subnet mask of 255.0.0.0, the router picks up a subnet of 128.0.0.0!!??

The only thing that has changed is that previous to last week, my ISP was giving out 58.x.x.x ip addresses, and a 58.x.x.x gateway address. However, as of this week, I am getting a 118.x.x.x ip address and a 58.x.x.x gateway, which is a little odd.

I'm wondering if somehow this is screwing up the DHCP on the modem and causing it to serve up an incorrect subnet mask to the router?

I think there might be a bigger problem however as even if I manually specify the correct IP details on my modem (& don't use DHCP), it still doesn't work (unless half-bridging requires DHCP in some way that I don't understand).

I tried obtaining IP details from the modem with my PC and I get the same result, so I don't think the fault lies with the router. I have tried resetting the modem to factory defaults and configuring it again from scratch to no avail. I am also running the latest firmware that I can find (1.19.02).

I've setup double-nat at the moment, which works ok, but is not what I want in the long term.

I've tried creating a post on the Linksys community forum, but for some reason it won't let me create a new post!

So, does anyone have any ideas?

Thanks,

Sam.


Last edited by Sambo on Sun Aug 09, 2009 10:54; edited 2 times in total
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MatrixAce
DD-WRT Novice


Joined: 31 Mar 2007
Posts: 14
Location: Singapore

PostPosted: Sun Nov 18, 2007 17:16    Post subject: Reply with quote
Have you set the AM300 using Bridge Mode only, DHCP Server OFF, and select your VPI and VCI correctly before you save the settings?

I've already deployed more than a few dozen sets of AM300 1.19.02 with WRT54GL 1.1 and doesn't have this issue.

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Sambo
DD-WRT Novice


Joined: 18 Aug 2007
Posts: 27

PostPosted: Sun Nov 18, 2007 21:41    Post subject: Reply with quote
DHCP server on the MODEM OFF?? I thought it needed to be on for half-bridging so that the modem could pass the Internet IP address details to the router!!??

But yes, apart from that, the modem works fine, I have connectivity as normal from the modem, but not via the router. As I said in my original post, this was all working fine until last week. There were no configuration changes on the modem or router before the failure, the only thing that's different is the ISP allocated IP address range change.
MatrixAce
DD-WRT Novice


Joined: 31 Mar 2007
Posts: 14
Location: Singapore

PostPosted: Mon Nov 19, 2007 0:45    Post subject: Reply with quote
Still the best way is to have the AM300 to set as "Bridge Only Mode". Then have WRT54GL configure for PPPOE or PPPOA settings as follow your ISP.

This way the router will receive the Public IP that the ISP have assigned to your Modem. Else if the DHCP on your modem is on, then it will not be passing Public IP to your router but Lan IP intead.

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WHR-HP-G54 w/ DD-WRT V24 RC5
Sambo
DD-WRT Novice


Joined: 18 Aug 2007
Posts: 27

PostPosted: Sun Aug 09, 2009 10:54    Post subject: Reply with quote
By the way, I did find a fix for this issue, it was due to a bug / oversight in the AM300 firmware. v1.19.04 of the firmware includes a fix for this issue.
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