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 Post subject: RTL8139
PostPosted: Mon Jan 21, 2013 7:34 pm 

Joined: Fri Nov 07, 2008 12:04 pm
Posts: 179
Location: Netherlands
Hello guys,

While building my lab for CCIE, I ordered a RTL8139C chipset based adaptor to be able to configure the topology with real switches with one 3750G as a breakout switch (I needed a network card which does not strip incomming vlan tags). Indeed using this new adaptor I can see CDP going both ways, vlan tags are not stripped anymore, all ok except that I'm not able to increase the MTU for more than 1500 on Windows 7 x64. I tried using windows command line utility, using registry and using different softwares, is impossible to move it to a value more than 1500. I can lower it, but not going higher than 1500.

My questions:
Anyone had any experience with this network adaptor under Windows 7 x64? Is a driver issue?
Is it mandatory to have MTU > 1500? Did someone had problems during studies because of MTU not being higher than 1500?
Can someone recommend me another cheap adaptor that will not strip vlan tags and in the same time allows an MTU higher than 1500, and has drivers for Windows 7 x64?


Later Edit: Maximum MTU 1500 is a limitation of the standard for FastEthernet. For anyone interested in the future, adapter used is: TRENDnet TE100-PCIWN PCI Network Card. I paid £2.99 on ebay for it.

 Post subject: Re: RTL8139
PostPosted: Fri Mar 01, 2013 11:12 am 

Joined: Fri Mar 25, 2011 11:09 am
Posts: 40
Glad you got your issue resolved.

Not being able to set your MTU shouldn't cause problems for MOST things. But there are certainly some issues that may arise. For one, if you create vlan subinterfaces on the routers you may find that your packets get dropped. Or if you do QinQ. Or you use the Direct VLAN subinterface method to connect to real switches.

Usually Windows can be tweaked to allow MTU modification. I haven't done it personally as I run Linux. But from what I have read either the device drivers allow it or Windows tweaking software can mod it for you.

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