Non-jumbo devices on jumbo LAN?

[Stilez]

Quick check on a simple situation:

I have a LAN on which almost all devices support 9k jumbo frames. The local network is Intel LAN on PCs and file servers, and 3 cheap-and-cheerful unmanaged Asus 1108N switches (don't need managed here and they work fast so outlay got saved for where it mattered more).

The LAN structure is: switch in each of 3 rooms (with their attached devices), and rooms linked using cat 6 fixed links between the switches. Most PCs are Windows 7, though a couple run flavours of Linux. Files of 5 - 100 GB routinely get sent across the LAN so jumbo frames make a difference.

While all PCs and servers are jumbo-enabled and tested (ping -f), there are a few devices where jumbo frames won't work - the external PPPoE internet router and 2 old HP LaserJets permanently plugged in are both 10/100, and probably one or two other devices that get plugged in randomly on the LAN wherever needed.

My question is, will these devices cope efficiently with the differing frame sizes? In other words,

1. Does networking hardware 'discover' and 'remember' that jumbo frames will work between devices (1) and (2) but a smaller frame is needed between devices (1) and (3), or will it keep hitting fragmentation slowdown and resends that I'm unaware of?
   
2. Is there a way to easily test what frame sizes are being used to transfer data from/to specific devices on my network, to verify what it's doing?

 

[thiggins]

My question is, will these devices cope efficiently with the differing frame sizes?

Nope. More info here Need To Know: Jumbo Frames in Small Networks

 

[Stilez]

My question is, will these devices cope efficiently with the differing frame sizes?

Nope. More info here Need To Know: Jumbo Frames in Small Networks

Read it, insufficient info to answer the question.

Switches here are a non-issue, they can all cope with jumbos. Packets are passing easily. The difficulty is I can't tell if packets are being resent in some cases (or how much), or if the devices sending them have recognised the maximum packet sizes for the various devices on the LAN and are being intelligent, sending 1500 to some devices, jumbo to others, based on prior fragmentation and discovery).

I also don't know enough about how modern drivers tend to handle situations where some of the IPs they send packets to have jumbo frames and some don't (ie if they detect and compensate for it, you'd think with companies of Intel's standing in networking that they would?).

The article you link to discusses switches rather than sending devices. The only devices that can create and transmit jumbo frames here are modern LAN connections (almost all Intel eg pro/1000 cards and onboard LAN etc) and their drivers, but I don't know if they detect and adapt to packet size limits on a per-destination-device basis, or if not what happens and how to test what's going on.

The article covers mixed networks but mainly in the sense of a subnet of jumbo devices, a subnet of non-jumbo devices, with a router linking them. The non-jumbo devices here arent doing that. They don't "talk to each other". They are devices such as PPPoE internet and old printers whose only actively used LAN interaction (via jumbo-capable switches) is almost always to/from jumbo-capable devices such as PCs and are used by all other PCs and servers.

The article does say a bit ("Once again, even if the link to the router is Fast Ethernet, the devices on the jumbo frame LAN can still access the Internet. The traffic from the jumbo frame devices to the Internet will just throttle down to a lower frame size utilizing Packet MTU Discovery, or the router will automatically fragment them") but doesn't say if this is a once-off discovery or if it's an ongoing source of lag and inefficiency.

Thats where I'm up to, help appreciated

 

[dnoyeb]

im going to go out on a limb and say its an ongoing source of lag and inefficiency. Otherwise, wouldn't adding jumbo frames 1 item at a time to a network be just fine?

I suspect each newly initiated connection will start with the full frame size, then throttle back.

But I don't know for sure.