Equipment (Modems,Gateways)
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SMCD3 Bridging - Do we need a mode change -

I notice several postings in this forum about setting the SMCD3 (and other Comcast modems) to bridge/passthru mode when utilizing static IPs, etc. Apparently this requires a support request, and is done at the headend. I was not aware that this was needed. Should we request this change?


We have an SMCD3 and a bundle of static IP addresses. We use a Netgear FVS336G router on the LAN side to manage a portion of our internal network. The Netgear WAN settings are currently configured to one of our static IPs and we can access it from the public Internet at that address. We use this for VPN remote desktop access, and can also access the Netgear web admin panel remotely.


The Netgear seems to operate properly as is, with one exception.  Speed tests ( run from a PC on the Netgear side typically run at a fraction of the speed as the same test when the PC is plugged directly in to the SMCD3.



SMCD3 <-> Netgear <-> PC ( = 6.49 / 6.27 Mbps down/up)

SMCD3 <-> PC ( = 86.84 / 12.42) Mbps down/up)


Could this speed difference be the result of the bridge/passthru mode setting? We have assumed that it was a quirk in the Netgear settings. Although the difference seems large, the LAN side has very light traffic, and users do not seem to notice any performance problems.


As a related question: When the bridge/passthru mode is set from the headend is it port specific? Although some of our internal LAN traffic goes through the Netgear, we also use the other 3 SMCD3 LAN ports for various devices - PCs, printers, etc. These pick up the default 10.1.10.nn addresses from the SMCD3 DHCP.





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Re: SMCD3 Bridging - Do we need a mode change -

Welcome Waldo3.  The test results that you posted point to a setting in the Netgear.  Most likely a QoS or 10BASE/Tx setting for the Netgear WAN port that is limiting your bandwidth.  


Bridge mode is a function of the Gateway and since you have fixed static ip's your gateway is not in bridge mode as bridge mode and permanent fixed static ip's are incompatible.  


Also traffic from the headend to the gateway follows the standard protocol rules established by IANA.  (ex: port 80 is internet. ) However you can set port forwarding rules in the gateway and the Netgear for specific applications and devices.  

(ex: remote access, security cameras.)


Thank you




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Re: SMCD3 Bridging - Do we need a mode change -

Comcast_John's above answer is completely wrong.


I realize this post is old, but it comes up on the first page of Google results, so I'd prefer people who want to do this be informed that it's doable (and very well should be- I wouldn't pay any business service provider who wouldn't let me put my own router on).


You CAN put the modem into bridge mode with static IPs.

I have this set up, using a pfSense instance as my router/firewall/NAT/IDS.
I have a Netgear gigabit switch downstream from that, and a Netgear wireless router on that (as just an access point, with firewalling and DHCP disabled, connected to the switch by one of its LAN ports, rather than its WAN).


To put the SMC D3G into bridge mode, what you need to do is disable the built-in DHCP server (in LAN > IP Setup) and disable the built-in firewall. You can then handle all of that through your router.


As far as your speed is concerned, some routers don't play well with the SMC D3G's default auto speed setting (in LAN > Switch Controls). You might try deselecting Auto on any ports you're using, and set them explicitly to Speed: 1000, Duplex: full, which forces it into full-gigabit. Likely, your router will have similar speed options, and you should set them similarly. If that doesn't work, then adjust your QoS settings. Normally, routers don't throttle your performance out-of-the-box.


Aside from the usual online speed tests, your best option is to install iperf. You can run it between to machines on your LAN, or over the internet. Normal average speeds over a gigabit LAN should be in the 940Mbps range. I average 941Mbps on the network I described above. Running iperf to a static ip which is then pointed via NAT to a Debian server on the LAN, I average 522Mbps, which is also normal.

Hope that helps,


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