Gigabit home network - jumbo frames? 1

[Nelviticus]

Hi there, my home network is currently centred around a fast ethernet (10/100) router/wireless access point. Most devices are connected to it via wired connections. My main PC has a gigabit-capable ethernet port and I have just bought a NAS which also has a gigabit ethernet port.

I am looking to add a gigabit switch, connect the PC and NAS to it then connect it to the router so that I have a gigabit network internally. However a customer review comment for the cheapo switch I'm looking at says that they don't think it supports jumbo frames.

Is this something I need to worry about if I want my network to function at top speed? The NAS definitely supports jumbo frames and I'm fairly sure the PC does, but I don't really know what they are and what difference they make.

Thanks

Nelviticus

 

[jimbopalmer]

Standards based ethernet has a maximum frame size of about 1518 bytes, there is some variation with VLANs and QoS.

As speeds increase, the processing time to do the error detection checksums on each packet can tax slow CPUs. Cheap gigabit cards offload a lot of this onto the computer's CPU, more expensive 'server' cards do the math on card.

One, nonstandard, approach to releiving the CPU bottleneck is to increase the frame size to around 9000 bytes, this cuts the math by 1/6. This is called Jumbo Frames.

Because it is not a standard, innocent equipment (your router, your switch, other computers, network printers, etc.) can become confused with Jumbo Frames, and refuse them as bad packets.


What you usually do is build a second network with all Jumbo Frame equpment, which carries the heavy load, and your primary network handles all the standards based communications with the world.

(Just a comment here that most home computers will only get about 100% to 200% speed up over fast ethernet due to Windows file handling and hard drive transfer speeds, followed by PCI bus speeds)

So, you COULD wire the NAS via a Jumbo Frame switch on a seperate network with a separate subnet and static IPs.

Or you could do without Jumbo Frames and have one network do all your traffic, simply.


I tried to remain child-like, all I acheived was childish.

 

[Nelviticus]

Thanks for the reply Jimbo.

So, if I've understood you correctly, if I attach the PC and the NAS to a gigabit switch I will achieve faster performance than via the router but unless switch, PC and NAS all support jumbo frames this will come at the price of increased CPU load?

I also take it that the reason for putting it all on a separate subnet is to avoid sending jumbo frames to the non-gigabit router?

All I'm really after is getting a higher-speed connection between the PC and the NAS while maintaining connectivity to all my non-gigabit equipment. I realise this isn't going to be anywhere near a 10x speed-up but even 2x would be a huge improvement.

Regards

Nelviticus

 

[jimbopalmer]

Yes, more load unless the NIC in the PC handles the checksum math.(offload is the usual buzzword)

Yes, not all equipment will choke on Jumbo Frames, some will send error messages asking for smaller frames, but equipment not expecting Jumbo Frames MAY choke, as they are non standard.

I would try it as all one subnet, with normal frames and if you get CPU loads above 75% then consider Jumbo frames or a better NIC.

http://www.intel.com/network/connectivity/products/pro1000mt_server_adapter.htm

I tried to remain child-like, all I acheived was childish.

 

[Nelviticus]

OK then, I will try your suggestion and see how it goes. Thanks for the advice!

Regards

Nelviticus