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router w/ jumbo frames yet?

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December 8, 2006 8:48:58 PM

Aside from the Asus RX314, is there another router with 9k jumbo frame support at this time?[/b]

More about : router jumbo frames

December 8, 2006 10:06:50 PM

For jumbo frames support, you generally need to be careful about hardware and firmware versions, etc., as most recent versions might have support but not older versions, but older versions can still be found readily in retail.

The Linksys RVS4000 is said to have support for 9K.
The D-Link DIR-655 is informally said to have support, but this is unconfirmed.

A better solution in some cases is a separate GbE switch of a revision supporting jumbo frames.
December 8, 2006 10:17:12 PM

I have a GS-108 that supports jumbo frames. The problem is my current router doesn't support them and in order for the router to uplink to the switch without problems I can't use jumbo frames in the LAN and expect good connectivity to the WAN.

cable modem <--> router <--> switch <--> PCs
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December 8, 2006 10:49:51 PM

What's the problem? My router doesn't support jumbo frames either, but I have no problems running a GS608 with jumbo frames on some NICs and not on others, connecting to the Internet from every point, transferring files from every point to every point, etc.

Path MTU detection should be taking care of the issues point to point.

The one problem I've noticed is connecting to my router's web configuration with jumbo frames enabled -- it just hangs. I believe this could be a problem in the router's implementation. There's an easy workaround of course -- use another connection or temporarily disable jumbo frames.
December 9, 2006 12:34:54 AM

My understanding is that if there is one PC that DOESN'T have JF enabled, the switch will drop the MTU size down to match the slowest device in the network. In other words, all PCs need to use a larger MTU size or else it automatically will drop to match the smallest MTU size.

Additionally, if all PCs are using JFs, but the router doesn't you'll get tons of dropped packets going to the router, no? And if the router won't know what to do with incoming packets either is my understanding. Can anyone comment? Can one use a standard 10/100 router with standard MTU size with a GS108 using either 4k or 9k jumbo frames and have both talk to each other without dropped packets?
December 9, 2006 3:20:29 AM

Quote:
My understanding is that if there is one PC that DOESN'T have JF enabled, the switch will drop the MTU size down to match the slowest device in the network. In other words, all PCs need to use a larger MTU size or else it automatically will drop to match the smallest MTU size.


Switches aren't typically that smart. They transmit different sized frames all the time -- i.e. ones smaller than 1500 bytes, and don't drop the global frame size down to the smallest size. All they'll do with an unsupported frame size is drop the frame.

I'm surprised that you're concerned about this, when you should just try it and see that it works fine. Perhaps the problem is that you have an older revision GS102 that just doesn't support jumbo frames? There's a support note on Netgear's web site that gives details on which revisions support jumbo frames.

Another test for the switch would be comparison with a direct wire connection. These will of course support jumbo frames, and gigabit will support straight-through in addition to crossover. Use static IPs. Wireshark and similar capture tools can tell you the frame sizes going across. This is also a good test for switch performance -- a definitive "wire-speed" reference in your environment.

Again, I've been running a mixed frame size environment for some time now, and verified sucessful transmission of jumbo frames where both ends support them, and auto-downshift to normal frames when one side doesn't, and shift back up where both do, etc.

Quote:

Additionally, if all PCs are using JFs, but the router doesn't you'll get tons of dropped packets going to the router, no? And if the router won't know what to do with incoming packets either is my understanding. Can anyone comment? Can one use a standard 10/100 router with standard MTU size with a GS108 using either 4k or 9k jumbo frames and have both talk to each other without dropped packets?


My DGL-4300 router could be considered a standard 10/100 which doesn't support jumbo frames for this purpose. It certainly doesn't support jumbo frames, and it only has a 10/100 WAN port. Jumbo frames will not go through it, yet I can surt the Internet, etc., just fine through it from this jumbo-frame enabled workstation connected to a jumbo-enabled switch. The magic of Path MTU Discovery at work..

Dropped frames in this way are not subtle, and even if they were somewhat subtle, I do enough performance measurements and wire captures that I'd know when they're happening non-insignificantly.

Most consumer routers will not support jumbo frames, and besides, even if they did, you can't exactly expect the whole Internet and all its routers to support jumbo frames. Case in point -- your router and mine are actually on the Internet, and mine certainly doesn't. To repeat -- so what if your router supports jumbo frames? The next hop probably won't. Routers supporting jumbo frames typically only matter for the switch part, within your local network. Here, a good switch by itself will work just fine.

There is a problem with routers which don't support jumbo frames, but it's not as bad as you think. The problem is generally avoided well with the Path MTU Discovery protocol -- where upon connection, each side sends its MSS as part of the message. The rest of the communication for that connection then uses a MSS size that's mutuallly supported. So, going across the router to the Internet typically works fine, because what's on the other side does not try to support jumbo frames, and sends that information across at connection time.

This breaks down when the other side actually supports jumbo frames, but a router in between doesn't. (Note however that local network connections on a local switch do not go through the router in this way -- only WAN connections do.) Well, there's supposedly a solution to this as well -- Black Hole Detection, but from what I see, it doesn't work all that well. It's worth a shot if you're having problems of course. Otherwise, there's the old fallback -- use standard frames! Really, this jumbo frame business is often more trouble than it's worth!
December 12, 2006 6:17:43 PM

Switched on both 4k and 9k jumbo frames behind my switch and everything works upsteam of the switch. Cool.

[code:1:9e7f110a16]cable modem <--> router <--> switch <---> PCs.[/code:1:9e7f110a16]
December 20, 2006 3:16:58 AM

Grats on getting it working.

A correction to my above post: I got the terminology wrong -- a part that I described is called the MSS option announcement / exchange -- it isn't Path MTU Discovery. An notable point here is that the MSS option works at the TCP layer. This is fine for the majority of what we do, but is not everything. UDP in particular will behave differently.

So there could be another potential "gotcha" lurking for UDP. Workaround as usual -- turn off jumbo frames if needed.
December 20, 2006 1:21:12 PM

I think you're going to have to dumb-it-down a bit. Are you saying that TCP traffic is fine w/ mixed mtu sizes compared to UDP traffic that isn't? I'm assuming this means UDP traffic behind the switch only?

I pretty much only use NFS and SAMBA behind the switch and they both work fine. Beyond the questions I asked, what scenario would one get into trouble using 4k frames behind a switch knowing that the router that is in front of the swtich can't use them?
December 20, 2006 3:02:38 PM

Don't worry, I was just making a technical correction and giving you a "heads up" in case you saw that applictions using TCP were working fine but not those using UDP in some cases. Most high-level applications should be TCP.

All traffic behind the switch should only depend on the locally-connected hardware and the switch, not the router. However, there is a possibility that some local UDP transfers would have problems when TCP wouldn't.
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