DHCP Issue

[bdrmachine]

A few times a year I lose internet connection from Spectrum cable.  My network is configured as follows:  Cable modem (EU2251 box) connected to a Asus RT-AC1200 V2 WiFi router then to a cisco SF220-24pr smart switch.  My main computer is hardwired to the WiFi Router.  When the network goes down (Even after several Cable Modem full power downs / resets) My WiFi router claims "Your ISP's DHCP does not function properly".  If I reset my WiFi router I get internet for about 30 seconds but it goes back out as soon as the WiFi LED's start to light back up.  If I connect a computer straight to the back of the cable modem that computer continues to work.  Spectrum swears up and down the problem can not be theirs if connecting straight to the back of their box gives internet service.  Nothing I seem to do helps and the problem goes away by it self if I wait a few hours or days.  I have changed out the WiFi router but no help.  I have reset all devices in sequence but zero luck,  I did go to the store (45 miles away) and  exchange the modem, twice.  Each time service comes back and routes thru all my equipment once installed.  What could be wrong with Spectrum's DHCP or my equipment.  How do I convince Spectrum to look into this issue?

[deanwebb]

My first thought was, "does the PC to the ISP modem work? If yes, not ISP problem". Not surprised that was Spectrum's answer.

What if you connect the SF220 directly to the ISP gear? Does that still work?

Wondering if a different brand of WiFi will work. Another thought would be to have some other device hand out DHCP addresses on your network and get the home gear to point at that device instead of the Spectrum one.

[bdrmachine]

It makes no difference if I connect the switch 1st or the WiFi router, neither one will allow internet.  I only have Asus WiFi routers but none work.  They are different models but all top expensive units / no cheapees.  Is there a way to configure the WiFi router to be the DHCP  server?

[Dieselboy]

With DOCSIS (cable modems) my experience has been that once the client device on the LAN side successfully requests a DHCP OFFER, all future DHCP packets are ignored. It means that one device can get a public IP from DHCP and a 2nd or more devices on the same LAN (broadcast domain) will not get any IP from DHCP.

I am wondering if either of the following are applying to your internet issue:
- DHCP lease expires before DHCP is successful
- or your router is trying to renew the current lease (as a unicast packet to the existing DHCP server) while the ISP is wanting your IP to change


A best next-step in my mind would be to capture the packets to and from your router some time before and after the issue. The router will try and renew the existing IP at when 50% lease time expiry. For example if the lease time is 1 day then at 12 hours you should begin seeing DHCP packets trying to claim or hold on to the current IP. 


I cant think why your internet would work for 30 seconds when you reboot your router. Again, packet capture might help. I make an assumption that your wifi router is working fine, since you replaced it twice and now using the 3rd.
If you can reproduce this then if you can SSH to your router and check some things out like:
ps - I think these routers run a flavour of openWRT and there is also Asus-WRT (Merlin) that you can load, but I had not needed to when I had an Asus router.

1) what is the router WAN IP (

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ip addr)
2) what is the router default gateway (default route) (

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ip route or

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route -n)
3)  Does the router have an ARP entry for the upstream gateway (

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arp -an)
4) can you ping the upstream gateway IP (I'd expect no if internet is down, otherwise go to #7)
5) if you clear the arp and then try pinging the upstream gateway again, does it work now?
6) if it now works after clearing ARP, is the mac address different now compared to what the ARP table showed earlier in #3?
7) if you can ping the upstream gateway when internet is not working, run a trace to 1.1.1.1 and see how far it gets

The last one (#7) would say to me that the issue is in the ISP cloud.

I had a similar issue with my internet connection years ago and the upstream gateway in the isp cloud was moving the IP and the MAC address was changing occasionally. This meant that after the IP moved, my router couldnt send traffic to it though the IP did not change, the ARP table on my router was wrong. The Cisco default ARP timeout was 1 hour if I remember correctly, so I changed the ARP timeout to 60 seconds and that was all that was needed to fix the issue for me. 

[bdrmachine]

Thanks for replying.  This is extremely intermittent but very frustrating.  This gives me some ideas were to look.


Thanks