MTU and Jumbo Frames questions.

[manowar]

Hi all,

I need a couple of things clearing up to put my mind at rest...any help would be great.

Firstly, I've just bought a new Asus GX1105N Gigabit switch that supports jumbo frames. Currently, its plugged into my Linksys 10/100 router, then my PC connects to the Asus switch - my LAN port and the switch are both gigabit AND according to the advanced settings for my PC port, there is an option to set Jumbo frames.

1. In order to reap the benefits of the jumbo frame facility, as my PC and Switch support it, should I increase the jumbo frame size to its max on my PCs NIC? In this case, 9014 bytes?

2. Also, my broadband is with Virgin Media. I've been fiddling with MTU sizes in my Linksys router settings, and there's an option to manually set it OR disable it. Does anyone know what the optimal MTU size is for Virgin Media? Or should I just disable it (defaults to 1500 I think).

3. Lastly, this is a trivial question and I think I already know the answer - but is the max MTU on an ethernet network 1500 regardless if its 100mbps or gigabit? Unless you have devices that support Jumbo Frames? (In which case its 9000)?

Cheers all.

 

[mikesgroovin]

1. In order to reap the benefits of the jumbo frame facility, as my PC and Switch support it, should I increase the jumbo frame size to its max on my PCs NIC? In this case, 9014 bytes?

Yes, if you wish. (9216) Since the PC sits on the downstream side from the cable connection, this should be fine. Do you have any other devices or nodes that sits on the new switch that don't support jumbo frames?


2. Also, my broadband is with Virgin Media. I've been fiddling with MTU sizes in my Linksys router settings, and there's an option to manually set it OR disable it. Does anyone know what the optimal MTU size is for Virgin Media? Or should I just disable it (defaults to 1500 I think).

The most efficient size for virgin media is going to be 1500 (as with any broadband connection) You can go higher but you are going to see transfers (mainly uploading) taking longer because of packet reassembly on the other end. I would just keep your MTU at 1500 on the gateway/router.


3. Lastly, this is a trivial question and I think I already know the answer - but is the max MTU on an ethernet network 1500 regardless if its 100mbps or gigabit? Unless you have devices that support Jumbo Frames? (In which case its 9000)?

Try not to relate MTU with the speed of the network. It's often considered over faster LANs but they are two totally different things. You can set it to whatever you want, but if they are larger than 1500 anywhere else and your sending packet is 3900 bytes, then the receiving end will have to defrag and reassemble the packet (which takes more time). This can be tricky especially over special connections that require more header room (VPNs, etc) If you dont know what you are doing leave it at 1500 on the device communicating over the internet and then use support for jumbo frames for anything else on the local side.

A larger MTU doesn't necessarily mean "faster networking". Typically, if the MTU and other variables are important to you in your physical network topology build process you will have to run tests with different packet sizes over and over to different devices to get a series of results... lay them out and figure out which devices communicate more efficiently given: packet loss ratios, packet sizes, speeds, etc. Even if your switch and PC have the average packet loss for a switch and the model NIC you have, you wont notice that much of a difference due to the speed. It will only occur every once in a while and you probably will only notice that the communication between you PC and switch is faster if you enable jumbo frames. If you connect another device that doesn't support jumbo frames to that switch, the communication between the 2 PCs should be fine too given that your packets wouldn't have much to fragment, but it would be better to setup that segment of the LAN with full jumbo frame support. Your jumbo frame supported PC would soon learn that the communication to the other PC that didn't have jumbo frame support would be more efficient at 1500 (or whatever the max MTU is set for on that node). It's part of the MTU path discovery, but thats getting a little further down the "tangent path" that I often take.

Hopefully your questions were answered

 

[bilbus]

Do you need jumbo frames? Usualy a waste unless you are moving many gigs at a time.

MTU for ethernet is 1500, anything larger will cause you problems.

If you want to go jumbo frames all devices on the segment need to be jumbofram capable (aka no 100mb switches)

I would stay away.

 

[mikesgroovin]

Do you need jumbo frames? Usualy a waste unless you are moving many gigs at a time.

MTU for ethernet is 1500, anything larger will cause you problems.

If you want to go jumbo frames all devices on the segment need to be jumbofram capable (aka no 100mb switches)

I would stay away.

The poster already indicated that he has a jumbo frame capable switch. And All nodes don't "need" to have support for it. Switches will learn paths and so do NICs. Ethernet over the internet uses 1500 (mandated by the IEEE) but for his LAN, using jumbo frames is fine. Recommending to stay away??? LOL, maybe in 2003 but jumbo frames are fine in LAN use.... have been for years.. *sigh*

 

[manowar]

Hi guys, thanks for the replies. Sorry for the belated response.

Basically, my gigabit switch (it does support jumbo frames) has three devices attached. My PC (gigabit port) my WD MyBook (gigabit port) and my PS3 (gigabit port).

The thing is, I don't know if my Mybook or PS3 support Jumbo Frames. All I know is that my PC does (it has the option in the NIC properties). From what I understand, if I were to transfer a file between say, my PC which supports jumbo frames and my MyBook (lets say it doesn't support them) - then would I see no increase in performance because the MyBook cannot receive packets greater than 1500 without defragmenting them (due to lack of jumbo frame support)? However, transfer TOO the PC would be faster because it supports jumbo frames and probably won't have to defrag packets as much?

mikesgroovin - thanks for your in depth response. It helped tremendously.

 

[mikesgroovin]

if I were to transfer a file between say, my PC which supports jumbo frames and my MyBook (lets say it doesn't support them) - then would I see no increase in performance because the MyBook cannot receive packets greater than 1500 without defragmenting them (due to lack of jumbo frame support)?

Correct. In fact, your PC, if tasked with the job of sending data to the MyBook (which does not support Jumbo Frames BTW... not even the world editions) would send at 1500 due to MTU discovery.

However, transfer TOO the PC would be faster because it supports jumbo frames and probably won't have to defrag packets as much?

No, unfortunately, you aren't going to notice an increase if you switch between jumbo frame or no jumbo frame support on the PC if the data coming to it is set at 1500 MTU. In theory, your logic makes sense, but that wont take place until it's past the physical layer anyway. And once you are past the physical NIC, the data won't be serial anymore, it will be parallel. It will be faster, but hardly in a sense where you will ever notice a difference.

mikesgroovin - thanks for your in depth response. It helped tremendously.

Sure thing. Sometimes I can ramble but I only do it in order to make sure that the technical detail that I'm trying to convey is clear (for the most part)

 

[manowar]

mikesgroovin, thanks for clearing things up.

Because the MTU is set to 1500 on my router, even if I have two devices on my LAN that support jumbo frames and are hooked up to my switch that supports jumbo frames - will the data be still set to transmit 1500, because the router is set to 1500? Or since the devices are attached to the switch, do they evade the routers MTU, and implement the jumbo frame facility allowing for transmission of larger packets? In other words, does the routers MTU take precedence? Or does the switch dictate the MTU to devices attached to it?

Sorry if that sounds a bit convoluted, I'm just trying to get my head round this.

See at the moment, my setup is this:

[CABLE MODEM] ----->[ROUTER]---->[Gigabit Switch(supporting jumbo packets)]----->[MY PC] (supporting jumbo packets)

 

[mikesgroovin]

mikesgroovin, thanks for clearing things up.

Because the MTU is set to 1500 on my router, even if I have two devices on my LAN that support jumbo frames and are hooked up to my switch that supports jumbo frames - will the data be still set to transmit 1500, because the router is set to 1500? Or since the devices are attached to the switch, do they evade the routers MTU, and implement the jumbo frame facility allowing for transmission of larger packets? In other words, does the routers MTU take precedence? Or does the switch dictate the MTU to devices attached to it?

Sorry if that sounds a bit convoluted, I'm just trying to get my head round this.

See at the moment, my setup is this:

[CABLE MODEM] ----->[ROUTER]---->[Gigabit Switch(supporting jumbo packets)]----->[MY PC] (supporting jumbo packets)

Yes, my understanding of your setup has been your true setup from the start of the thread. Your gateway/router doesn't even come into play considering the path the packets take to get from the MyBook - to the switch - then to the PC (and vise-versa). Your gateway/router will evade all LAN-based traffic unless you use an external IP or DNS to communicate. The switch does all of the "deciding" and eventually (after the first packet) your PC will know that all traffic to the MyBook needs to be at 1500 MTU. The MyBook NIC firmware does does support jumbo frames so it will always communicate via 1500 MTU (1492) no matter where the data is going to.

 

[manowar]

Cool, cheers mate.

So yeah, basically if two devices that are jumbo packet compatible which go through the switch, they will utilize the facility and go beyond the 1500 MTU that the router abides by?

 

[mikesgroovin]

Correct