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SB8200 flips from bridge to gateway when cable fails

C
The Many
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Greetings and salutations —

 

I have two Arris modems — an SB8200 with WOW/RCN/Astound service and a T25 with Comcast/Xfinity service connected to a router.  The router has multi-pathing enabled and balances remote destination addresses across the two WAN connections.  Under normal circumstances, this works well.  If Comcast’s service fails, the T25 blinks out, the router removes it as a routing option, and life goes on.  BUT, if WOW service goes out, the SB8200 doesn’t blink out with it — instead it switches from being a bridge to WOW’s network to being a gateway to nowhere.  It leases my router a new address from its built-in pool and carries on.  This is an incredibly annoying behavior because it results in external addresses being balanced between the remaining WAN connection (Comcast) and a black hole (WOW.)  Devices on my internal network can get to e.g. ibm.com but not e.g. google.com.  Is there a way to force the SB8200 to function as a bridge when it looses signal?  Is this a carrier provisioning choice?

 

Thanks!

 

Chris

Best answer by plemans

The sb8200 has zero router capability. Meaning zero capacity to running a dhcp server. 

Its a modem only device. It doesn’t “become a gateway” when the internet is down. 

So I’m not sure what you have it connected to but I’d imagine thats where I’d start. 

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C
The Many
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  • cwickleinThe Many
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  • The Many
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  • May 18, 2023

For the sake of completeness, an obvious work around is to temporarily disconnect the SB8200, but that requires touching it and defeats the point of having redundant connectivity.  I suppose another option is to handle leases for RFC 1918 address as a special case, but the extra complexity is a last resort.

plemans
Juggernaut
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  • plemansJuggernaut
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  • May 18, 2023

The SB8200 is a modem only device. it doesn’t have a bridge mode. 

Are you using it with wan port aggregation enabled? IF so, I’d disable that and only have 1 connected as I’ve seen issue with wan port aggregation on modems. 

Current setup: Netgear CM3000-->Netgear RBE973S-->2.5gig tp-link switch, Xfinity 2.4gig/350mbps
C
The Many
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  • cwickleinThe Many
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  • May 18, 2023

Nope, I’m not doing link aggregation with the SB8200; just one coax to the outside and one Ethernet cable to the router.

 

When you say the SB8200 doesn’t have a bridge mode, I understand that to mean it doesn’t expose an option to switch between bridge and gateway mode because it’s only over operating as a bridge.  The crux of my problem is that when the SB8200 looses its connection to the cable company, it ceases operating as a bridge and effectively becomes a gateway.  (Even in its normal mode of operation when it’s functioning as a bridge, the SB8200 has an interface on 192.168.100.1, but when it looses signal, then the DHCP server wakes up and it becomes a (useless) gateway.)

plemans
Juggernaut
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  • plemansJuggernaut
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  • May 18, 2023

The sb8200 has zero router capability. Meaning zero capacity to running a dhcp server. 

Its a modem only device. It doesn’t “become a gateway” when the internet is down. 

So I’m not sure what you have it connected to but I’d imagine thats where I’d start. 

Current setup: Netgear CM3000-->Netgear RBE973S-->2.5gig tp-link switch, Xfinity 2.4gig/350mbps
C
The Many
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  • cwickleinThe Many
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  • The Many
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  • May 18, 2023

The SB8200 is a bit more than just a bridge between the Ethernet on one side and the cable network on the other.  Even under normal circumstances when it’s acting as a bridge, it’s also operating a web server on 192.168.100.1 (as does the T25.).  If you’re trying to get to two different web servers on two different devices on two different interfaces, both using the address 192.168.100.1, then it gets a bit trickier.  Presumably .1-.10 are reserved for such applications, and the modem offers DHCP leases beginning at 192.168.100.11, but only when it’s lost signal, and even then possibly due to a provider provisioning option.  That’s where I don’t have much insight and the basis of my question.

C
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  • cwickleinThe Many
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  • May 18, 2023

My guess is that the SB8200 is providing a DHCP response so that the router will keep its interface on the SB8200 up and therefore have an easier time getting to the web interface.

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