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si-the-pie
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DHCP in modem mode

Hi All,

For some reason I can no-longer get a DHCP address when my hub3 is in modem mode. It was working for months up until a couple of days. Virgin support don't really seem geared up to support modem mode and said they would escalate and email me (nothing received as yet). I have a Cisco router connected to port on of the hub and according to debug I can see the Cisco requesting a DHCP address, but nothing is coming back. I've rebooted everything and also changed the cable between the router and the hub. I suspect a problem with the upstream DHCP server, but have no way of finding someone who can check it for me.

Any ideas? Thanks, Simon

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Adduxi
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Message 2 of 10
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Re: DHCP in modem mode

It needs  to be done is a certain order, as the Hub will hang on to MAC addresses at times.

Set Hub to Modem mode and confirm.  Once done switch it off

Power up your Router in DHCP mode and leave it switched on.

Connect the Router's WAN port to the Hubs LAN using as known good Cat 6 cable.  Only one port is active in Modem mode, so there should be no other ethernet cables attached.

Power on the Hub and wait for it to stabilise.

You should be good to go.  This only needs done the first time you attach a new Router.  Once the new Router MAC is stored it will work with any power on sequence.

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si-the-pie
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Re: DHCP in modem mode

No joy I'm afraid. Next step I think is to connect my laptop directly to port 1 of the hub and see if that gets a DHCP address.
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Tudor
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Re: DHCP in modem mode

Excellent idea, but be sure to power off hub before connecting the PC and power it on when the PC is up and running.


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legacy1
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Message 5 of 10
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Re: DHCP in modem mode

If that works copy the MAC to your Cisco router and see if that works if not on your laptop that work go to Registry Editor
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\key that gets the WAN IP
set
DhcpConnForceBroadcastFlag to 1 and reboot to test DHCP by Broadcast should your Cisco router do DHCP by Broadcast and not by Unicast

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si-the-pie
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Message 6 of 10
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Re: DHCP in modem mode

Laptop works fine. If I switch the hub3 to routed mode, the cisco router is able to pick up a DHCP address from the hub (albeit a private IP). Think this proves the cable and the Cisco router are fine, especially given it was working for months up until a few days ago. I've even tried defaulting the hub. Still no joy. Fairly convinced my the cisco MAC address is not playing ball with the upstream DHCP server. Unfortunately, I can't change the MAC address on the cisco as it's not supported on this model (897). I have an cisco ASA lying around that I will try in the next day or so.

Cheers, Simon

 

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si-the-pie
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Re: DHCP in modem mode

Actually, I've fixed this now. I'd forgotten my Cisco router also has gig WAN port (I was using an interface VLAN on the switch block). The WAN port has a different MAC address, and immediately burst into life, further supporting the fact that my Interface VLAN MAC address had been blocked. No idea why. If anyone knows please post here. At least I know this can happen now, and the good news is because this is a WAN port I can manually reconfigure the MAC address if it happens again.
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legacy1
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Message 8 of 10
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Re: DHCP in modem mode

What if you put the MAC you think is blocked on your laptop NIC and connect it to modem mode?
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Message 9 of 10
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Re: DHCP in modem mode

The VM CMTS only keeps one MAC address for your service. If you change to a different MAC address without the VM hub being the last device powered on you will never get a new DHCP lease.  I have changed my router numerous times and EVERY time I have had to fully initialise my router with the VM hub switched off and then power on the VM hub. I have never had any trouble following the correct sequence. You can ‘fudge’ it by copying MAC addresses, but that’s not good network etiquette.


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si-the-pie
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Message 10 of 10
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Re: DHCP in modem mode

According to what I've read on wikipedia the CTMS forwards DHCP broadcasts using IP helper, as unicast, to regular DHCP servers. I'd be interested to know what the policies of those servers are if anyone knows? i.e. in terms of lease, and DHCP options?
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