Can't change nic mac adress without modem lights



  • If I change the mac address on the adapter on the nic, my modem connected to it, the lights for LAN will flash for a few seconds then off.

    Cycle repeats nonstop.

    Modem is a sb5101.

    Anyone else have a similar problem?


  • LAYER 8 Global Moderator

    Normally if you change the mac of the interface connected to a modem, you need to power cycle the modem to connect tot he new mac.  I know this is how it on on my current SB6120, and on the one before that which was also a motorola SB51xx series I don't recall if was 5120 or just 5100, etc. and then even the one before that was the same way.

    http://homepage.ntlworld.com/robin.d.h.walker/cmtips/swap.html
    Cable Modem Troubleshooting Tips

    Swapping computers on the cable modem

    Most domestic cable ISPs (including NTL, Blueyonder) have the cable modem configured to recognise only one client PC, by the MAC address of its network interface. Once the cable modem has learnt the MAC address of the first PC that talks to it, it will not respond to another MAC address in any way. Thus if you swap one PC for another (or for a router), the new PC (or router) will not work with the cable modem, because the new PC (or router) has a different MAC address to the old one. To reset the cable modem so that it will recognise the new PC, you must power the cable modem off and on again.



  • Yeah, I know that though. Power cycled the modem several times. Although when what you descrbes happens, the modem will happily LAN conenct to the pc, but won't allow Internet.

    I also found that out when I was messing with my old sb6120.


  • LAYER 8 Global Moderator

    I have seen where when I reboot my sb6120 that it likes to hand out 192.168.100 addresses.. So maybe that would explain no internet.. I would suggest disconnect your device, change its mac – power cycle the modem and let it finish its sync and powerup etc before connecting anything to it.

    Also what mac you using?  Sure its a valid one, sure nobody else on your isp network could be using it?



  • Nah it's not the the things you mentioned. I've had that problem before, the link light is always blinking when that happens though.

    Here the link light blinks 3 times, pfsense then says disconnected. My log gets spammed for my modem of disconnects.

    It doesn't even get a change to negotiate an ip address.

    This only happens when spoofing. Like if I say hook my laptop up to the modem when it boots up, and then plug it into my router with no spoofing, I won't get an IP but it will happily communicate via the LINK and it will flash a lot.

    Here if I spoof regardless, it just blinks and then stops.

    Modem log is spammed with ethernet disconect and reconnect.

    Also tried lots of random macs, same problem… I think the problem is with pfsense. (I am rebooting the modem after pfsense is done loading)


  • LAYER 8 Global Moderator

    "(I am rebooting the modem after pfsense is done loading)"

    Disconnect the MODEM from pfsense before you change the mac..



  • OFC.
    It still doesn't work.



  • I have the exact same error too. Changing the MAC address will cause the interface to go up -> down -> up -> down. When I remove the interface mac address, it is all normal again and I notice the interface MAC has changed to what I want it to be. See if that is the case for you!


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