Highlighted

DHCP won't release cached addresses - how to clear cache

I changed the LAN IP address scheme on my Technicolor router. It is still assigning addresses from the old leases for devices that had an old lease. The lease time is set to a day, and was set to an hour before. This happened four days ago. I have rebooted via interface menu command at least three times.

 

How can I get the router to stop re-assigning, validating, confirming, assigning or whatever the old leases?

 

7 REPLIES
Sharer

Re: DHCP won't release cached addresses - how to clear cache

Have you tried to initiate a release/renew cycle from the client side? The client may be the one still holding the address... may not be requesting a new lease because it's locally stored assignment has not expired yet.

You may be able to accomplish it with something as simple as a power cycle of the device. Not a restart... but a full power down (the goal is to flush caches). May need to more directly prod it to renew/rebuild the connection in some cases though.

For a Windows system, you can try a "forced" renewal from an admin cmd prompt with a simple command:

Search for the phrase "cmd", and you should get a hit on the command file. Right click it and Run as Admin. This should open an elevated terminal style window with a blinking cursor to input text commands directly. Type the following phrase, and press the enter key:

ipconfig /renew

This should make Windows renew DHCP leases for all tcp connectors on that system. Sometimes you may need to use the /release switch first, then type it again with /renew for the more stubborn ones. Type the word "exit" and press the enter key to close the command window.

For phones and such, you may need to forget the network connection and then recreate it again.

Re: DHCP won't release cached addresses - how to clear cache

Yep, I've tried that.

I've tried power cycling physical devices like my Ooma Telo and Honeywell thermostat.

I've tried soft settings on devices that have a half step - like my smart phone I "forget" the wifi network (maybe even add a reboot) and then rejoin the WiFi network.

On a Windows workstation I've tried "ipconfig /release" followed by "ipconfig /renew" and also with a reboot in between.

The thing that's annoying is that all of these devices come back afterwards and have "no Internet" type connections. They all list a 192.168.x.x (old and same individually as before) address yet have the 172.x.x.1 (new) gateway and the Windows box lists the 172.x.x.1 router as the DHCP server. Which leads me to believe that the router has cached the DHCP lease for the IP address from before the reconfig and re-issued it based upon the MAC.

 

My next step is to try to power off the router and let it sit for five minutes, later this evening when none of the things I've been able to hard code IP addresses are using it.

 

Since phone support is overloaded and the chat bot is broken, I guess my next step is I plug it into 240v three phase and replace it on the lease/maintenance plan.... Smiley Happy

Established Sharer

Re: DHCP won't release cached addresses - how to clear cache

During setup you probably told your router to get its parameters from the network modem.  The modem usually tells the LAN router to be 192.168.0.1 and assigns devices going up from there.  That's not ideal, to keep Apple devices happy DHCP should assign the last octet starting at 11, NOTat  1!

If you are renting your modem and router package from Spectrum, they often lock the LAN address values so a user can't load an IP address that would be harmful to other network users.

Unplugging the router's power supply or coaxial cable  does not erase the stored MAC IDs.  You need to force a full factory reset by holding the the router's reset button for at least 15 seconds while it is powered up.  THAT will erase all volatile data and reload the factory defaults.  

Sharer

Re: DHCP won't release cached addresses - how to clear cache


@karlbeckman wrote:

During setup you probably told your router to get its parameters from the network modem.  The modem usually tells the LAN router to be 192.168.0.1 and assigns devices going up from there.  That's not ideal, to keep Apple devices happy DHCP should assign the last octet starting at 11, NOTat  1!

If you are renting your modem and router package from Spectrum, they often lock the LAN address values so a user can't load an IP address that would be harmful to other network users.

Unplugging the router's power supply or coaxial cable  does not erase the stored MAC IDs.  You need to force a full factory reset by holding the the router's reset button for at least 15 seconds while it is powered up.  THAT will erase all volatile data and reload the factory defaults.  


Good call... completely overlooked the whole ARP cache angle.  uggh...  so easy to forget the simple approaches and wind up overcomplicating things sometimes.  My bad.

 

IDK how you guys keep it all together in  the field with all the shifts in tech and software these days.  I got my fill of it during the Vista/Win7 transitions and bailed.

Established Sharer

Re: DHCP won't release cached addresses - how to clear cache

IDK how you guys keep it all together in  the field with all the shifts in tech and software these days.  I got my fill of it during the Vista/Win7 transitions and bailed.

 

If you're truly working in the networking field, it's not TOO bad -- you talk with your peers, you follow dedicated forums/wikis/chats, and you have the benefit of seeing a dozen or more similar problems each day.

 

 

Established Sharer

Re: DHCP won't release cached addresses - how to clear cache

@FrustratedCust  For future reference, this forum is peer-to-peer support, which means that we are just customers like you, not Spectrum employees.  But there's a lot of repetition in the user problem postings and some of us learn pretty quick what the classic 95% solutions are.  The good news is that none of us forum contributors has to go out and do the actual repair work, whether our hunches are right or wrong. The even better news is that some of the field techs ARE getting better, learning to look beyond the minimum time and effort repairs to something that will actually solves the customer's problem.  

Now if Spectrum 's management would just realize that happy customers do pay their bills and don't badmouth your products or services as much, things in the field would improve more.  They don't have to be a cable TV and internet company with the worst or second worst customer service reputation in the entire country.  But as  long as their business plan views every service truck roll as just another expense to be avoided they are shooting themselves in the foot at least once every hour! 

Apologies for the rant about perceived quality, you've surely heard it before in your field as well.  In the future please stop by often to contribute some of your decades of IT wisdom and experience here in the forums.  Forum folks and our fellow users appreciate the help. 

Re: DHCP won't release cached addresses - how to clear cache

@karlbeckman - Wasn't me asking, but I completely agree on the support point. The people I am paying money to wouldn't even communicate with me when I tried to use phone (first line) and online chat (second line). We should all thank the contributors to peer forums and open source for caring and moving the line forward!

 

On a different note - my original issue Smiley Happy (And to clarify, the DHCP problem is on the LAN side, not WAN side). I set the DHCP lease timeout to the minimum of 2 minutes and am able to get correct leases now. I have to go through the refresh-fail cycle once or twice a few minutes apart but eventually I am getting DHCP from the new pool. Worked for my mobile phone, a workstation, and a smart home device. Why it didn't work on hour or day I don't know. Why it would honor and refresh an address outside the assigned space (and it wouldn't put it in the connect device list) I don't know.