Cablemodem assigning private IPs

GotNoRice

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I have a Motorola SB6121 cablemodem connected to a custom PFsense router.

The issue is when the internet is being somewhat spotty, occasionally it might disconnect for 5-10 seconds.

What usually happens is the modem has a T4 timeout and reboots itself. After that, if the internet is still down it will assign a 192.168.100.x IP to the WAN port of my router instead of a public IP.

The problem then is that the internet will come back up, but there is a delay before it starts working again because the WAN port of my router still has a 192.168.100.x private IP address. I can either log into the router and force it to renew it's DHCP lease or wait an additional few minutes for it to occur automatically.

This is extremely annoying because the disconnects themselves are often very short, often so short that I don't even get disconnected from online games. But once the modem assigns the router a private IP, the internet will be down for minutes at least.

I have a dynamic ISP IP address, but it stayed the same for years. I tried entering the info manually as a static IP in my router and the results were great. Since the router was never being assigned a private IP, the internet would always come back up instantly once the disconnection was over. Unfortunately after a few days my internet stopped working, and after switching back to DHCP again it came back but it had a different IP. Apparently because I wasn't constantly refreshing my DHCP lease anymore, the IP was dropped, so I can't set it as static for more than a few days at most.

Any ideas?
 

bloodypulp

Gawd
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May 17, 2010
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leave it on DHCP, you caused an IP address conflict. set your router to auto-renew the WAN port more often. if your modem is frequently experiencing timeouts, contact your ISP. it should rarely occur.

when you were on Static, the ISP's DHCP server saw that your previous IP address was unassigned, so it assigned it to someone else. at that point, you had to revert to DHCP to get a new address.
 

GotNoRice

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leave it on DHCP, you caused an IP address conflict. set your router to auto-renew the WAN port more often. if your modem is frequently experiencing timeouts, contact your ISP. it should rarely occur.

when you were on Static, the ISP's DHCP server saw that your previous IP address was unassigned, so it assigned it to someone else. at that point, you had to revert to DHCP to get a new address.

I set it as static mainly as a test, I had no expectations of it working as a long-term solution. I actually have a Comcast Business account, which typically issues static IPs. Unfortunately the only way I could use my own modem instead of their SMC modem/router combo unit was if I used a dynamic IP. The SMC combo unit they offer has no way to disable the internal DHCP/NAT and I didn't want to double-nat just to be able to use my own router (which I make extensive use of) so that's how I ended up with a dynamic IP. Still, the fact that the IP hadn't changed since ~2010 or so did make me wonder if it was somehow tied to my modem due to having a business account, obviously that was not the case.
 
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If you assign your usable Static IP to your own router, place it in the correct subnet, and use the Static Gateway IP (one above your Usable IP) you aren't going to be subject to the SMC's internal DHCP/NAT.
 

GotNoRice

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If you assign your usable Static IP to your own router, place it in the correct subnet, and use the Static Gateway IP (one above your Usable IP) you aren't going to be subject to the SMC's internal DHCP/NAT.

The SMC is long gone at this point. For the brief time I used it, it didn't handle heavy torrenting nearly as well as my previous Motorola SB5100, so I got a SB6121 (was also moving to Docsis 3.0 at this same time). The modem has been great other than this private IP issue. The real question is why can't they let me use a Static IP with a SB6121? :confused:
 

hawk82

[H]ard|Gawd
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I have a Motorola SB6121 cablemodem connected to a custom PFsense router.

The issue is when the internet is being somewhat spotty, occasionally it might disconnect for 5-10 seconds.

What usually happens is the modem has a T4 timeout and reboots itself.

[snip]
Any ideas?

Why not contact Comcast to get the T4 issue fixed?
 

bloodypulp

Gawd
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May 17, 2010
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903
you can ask Comcast to change your modem to operate in Bridge mode, so your Pfsense can handle the routing duties.
 

Ehren8879

Supreme [H]ardness
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Sep 24, 2004
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The 192.168.100.x address is assigned by the modem itself. It isn't because of an embedded router in the modem, but more of a management mechanism while the modem is ranging, eg rebooting.

I'll have spin up a pfsense image in the lab tomorrow and see if I can replicate the lease time behavior. Normally a CPE gives up its private addy for a public assignment quite quickly.
 

GotNoRice

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Why not contact Comcast to get the T4 issue fixed?

I've had them over several times so far. And being a business customer, I even get same-day service most of the time. The problem is always that the disconnections are intermittent. It might only occur a few times per day. They run their tests, declare everything is fine, and leave. They don't seem to have any ability to solve problems unless the problems occur when they are there, which is about as likely as scoring a bulls-eye on a dartboard while blindfolded.

Last time, they said they didn't find anything wrong but that they did "tighten up" some connections out at the main box. That did seem to make the disconnections less frequent, but then again trying to identify patterns in randomness an exercise in frustration, and ultimately I still experience disconnects.

When I had it setup as Static, I found that the disconnections were often short enough that they didn't even disconnect me from online games or VoIP phone calls. But using DHCP, once I get a ghetto ass 192.168.100.x IP I'm fucked for ~5 minutes unless I login and manually renew the lease.

At least the DHCP issue is something I potentially have control over, and beats praying to the Comcast gods that they send me someone who can actually identify the problem. They threatened me with a $99 fee if they come out again and don't find anything wrong :rolleyes:
 

Ehren8879

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I don't know if this will definitely work, but if you apply an ACL to block DHCP on your pfSense WAN from range 192.168.100.0/24 then when your connection drops this should prevent the modem's internal DHCP server from being able to offer your router an IP.

Your public IP DHCP offer/assignment will likely come from a range other than this network (a CMTS IP). So, when your lease expires you should still be able to request an IP.

What you may lose is the ability to log into your modem's information page while it is rebooting. Once the modem comes online you should be fine though.
 

Dead Parrot

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Mar 4, 2013
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If you can, set the pfsense router to log the disconnects. Having a list of disconnects with date/time stamps over a several day period may encourage the tech to do a bit more then "tighten some connections". Is your modem on a UPS? Any chance the modem or its power supply is starting to go bad?
 

m1abram

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Mar 15, 2002
Messages
3,175
Following because I experience the same behavior on Comcast with my pfsense. I have had Comcast out several times and they just tighten connectors. I even show them their horrible wiring jobs at the poles all over the neighborhood but they just shrug. One pole has over 100 feet of coax just coiled up at the base of it in the woods. And yes that cable is connected and I assume used by the house near it. Most of the poles the coax has no strain relief and is just hanging from its connection and blowing in the wind.
 
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