DM200 has serious flaws in modem only mode resulting in port blocking or something

[paranoidsystems]

Right, I am beginning to lose my mind with Netgear home products.

I purchased a D7000v2 that has had to go back as it was constantly crashing at night and didn't support nat loopback. crazy for what it costs. So I decided I would not be buying another Netgear router so I got myself a Linksys wrt1900acs but I still needed a VDSL moden....so I was forced to get a DM200. the DM200 has loads of good reviews and seems a great solution for the cost.

So I have my wrt1900acs all setup and until my DM200 arrives I set my D7000v2 into bridged mode and I setup my connection in there as per isp, vlan 101, log in details etc. adn everything works perfectly. I can watch netflix on my firestick.

now my DM200 arrives, so I box up my D7000v2 ready to send back to amazon and setup my DM200 (update to the latest firmware) and stick in bridge mode with vlan pass-through. wait a few minutes and everything works. excellent. Then I fire up my firestick and start netflix and low and behold the app wont load.....along with BBC iPlayer....strange.....I check my router....nothing has changed...I have spent 2 days resetting devices re-flashing devices thinking I am going mad...I am sys admin for a large European shoe brand I should be able to get my home network running.....after approaching the issue device by device it would seem the DM200 is at fault. if I swap it out with another VDSL modem in bridged mode then suddenly netflix etc on my firestick works......so I am thinking I must be mad....it's impossible....so I unplug everything. set the DM200 to modem/router mode enter isp details. plug in a wireless AP and netflix works again......so I get my D7000v2 out. set that up as a router adn plug the DM200 (again in bridge mode) into it and bam....no netflix on the firestick.....

so am I going mad here?????? there should be nothing taking place on a bridge modem that blocks ports or shapes traffic etc but clearly when this DM200 is bridge mode something is taking place.....I have found quite a few posts of people reporting this exact same issue......

can anyone else with a DM200 connecting to VDSL in bridge mode confirm this issue? for me it seems that my firesticks (and some of my android devices) cannot access Netflix or BBC iplayer when the DM200 is in bridged mode. I am really annoyed that I have purchased 2 Netgear products this year that have both been let down by software bugs/omissions with no fix?

perhaps there is a hardware revision of the DM200 and I have old new stock or something? 

this whole experience is the exact opposite of the stacks of Netgear switches we have on sites around the world. 

[paranoidsystems · Accepted solution]

Right. after hours and hours of testing this I can confirm the issue. it is an MTU issue. I can replicate it using my DM200 and my D7000v2 in bridged mode. If I put either device in bridged mode and let the router auto the MTU it settles on 1492. I tried several routers and even tried the D7000v2 in router mode and DM200 in bridge mode and the router settles on 1492.

I quick MTU test gets me poor results

ping www.yahoo.com -f -l 1492

until I lower the MTU all the way 1400 and then like magic everything works fine! I can use Netflix iPlayer etc. How is a non-IT user ever going to find this out? I just wish I hadn't been so stupid and tested MTU first but such is life. 

now my real question is why on earth is this happening? and how come my open reach modem and my vigor 130 will settle on 1492 but my DM200 or D7000v2 in bridged mode will not? This has to be some sort of software bug in the bridging code?  I can only assume this has something to do with the way these things are bridging the interfaces in their OS.

Anyway. this really is a serious flaw that will really mess up the average users day and I know has frustrated me for 2 evenings. I plan on flashing this DM200 with OpenWRT and seeing what happens when I bridge it manually. at least then it can have a use.

[paranoidsystems]

I am replying to my own thread now. After extensive testing, I am pretty certain this DM200 is clearly doing some sort of routing and appears to be limiting certain ports coming into the network. This really is not the way a bridge device should behave. I can only assume that there some sort mac routing/spoofing nonsense taking place. What I don't know is if every DM200 does this. I have gone back several firmware versions and the results are the same.  Perhaps there are certain units with a change in design that is not accounted for with certain firmware? just crazy to me that I can swap in a bridge modem from another manufacturer and with no changes in my router that suddenly everything works.

[paranoidsystems · Accepted solution]

Right. after hours and hours of testing this I can confirm the issue. it is an MTU issue. I can replicate it using my DM200 and my D7000v2 in bridged mode. If I put either device in bridged mode and let the router auto the MTU it settles on 1492. I tried several routers and even tried the D7000v2 in router mode and DM200 in bridge mode and the router settles on 1492.

I quick MTU test gets me poor results

ping www.yahoo.com -f -l 1492

until I lower the MTU all the way 1400 and then like magic everything works fine! I can use Netflix iPlayer etc. How is a non-IT user ever going to find this out? I just wish I hadn't been so stupid and tested MTU first but such is life. 

now my real question is why on earth is this happening? and how come my open reach modem and my vigor 130 will settle on 1492 but my DM200 or D7000v2 in bridged mode will not? This has to be some sort of software bug in the bridging code?  I can only assume this has something to do with the way these things are bridging the interfaces in their OS.

Anyway. this really is a serious flaw that will really mess up the average users day and I know has frustrated me for 2 evenings. I plan on flashing this DM200 with OpenWRT and seeing what happens when I bridge it manually. at least then it can have a use.

[michaelkenward]

No problems here.

Remember, when replacing one component in the network, you cannot just swap one thing out for something else.

For example, the router behind the modem has to forget any bad habits it picked up when it worked alongside a different modem.

Reset everything and start from scratch. And it is important to do everything in the correct order.

[paranoidsystems]

I spent all testing yesterday. Where possible I reset and cleared arp chache etc between each test. And no matter what when I replaced either my dm200 or my d7000v2 as the modem in my setup everything works again. I ended up testing 6 VDSL modems (2 drayteks, 2 Linksys an open reach and a billions, all in bridge mode) yesterday and the only devices that required MTU changes in the router were the two netgear products I have. It’s completely insane. They are both going back to amazon now.