Jumbo Frame question

Hi all, I have a general question about this jumbo frame issue. I can install and setup basic lans/vlans etc so maybe I just know enough to be dangerous :slight_smile: . My understanding it that if the hub is connected to even a dumb unmanaged switch it should never 'see' jumbo frames on the network unless it's destined for the hub. I can't imagine any device produces a broadcast jumbo frame, but can they? And what device (sans a PC connecting to the hub) would need a jumbo frame for data to the hub? Or is there something else I'm missing? Thanks for any insight.

Hi @wecoyote5

I'm new to networking too. Here is a link to a brief article that may help. See the section, "What's the Catch?"

What are Jumbo Frames and How Do You Use Them? (techjunkie.com)

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Some devices just don't handle jumbo frames, HE is one of them. Honestly in a home environment they aren't necessary and honestly as a network engineer we really don't use them in larger networks as they really aren't necessary unless you're dumb enough to make a 2000 node network flat. LOL. Easier just to leave it off and avoid the headaches.

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Technical network engineering question for you Rick. For HE to receive a jumbo frame wouldn't it be true that (1) jumbo frames would need to be enabled on the network, and (2) some other device would need to be sending jumbo frames to specifically to HE? Or is there some sort of broadcast that uses jumbo frames?

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Do u have to turn them on at the netcard and the switch or only the switch. Because i just noticed on my managed 10g switch i have jumbo frames on the two nas , mail server and media/plex server ports.

The hubitat is behind a pair of moca connectors and 2 switches away. Pretty sure jumbo frames are not getting to it somehow.

Pretty sure every interface involved has to support jumbo frames. Routers, switches, NICs, etc.

Hi guys,

Thanks for the feedback and the article, it was a good read; and I agree that they're not needed in a home network environment (and I have them disabled). My question really isn't about the effects it has on the hub, it's more of a question as to how a jumbo packet, even if they're enabled, can even 'get' to the hub as the switches should be only be passing on packets destined for the hub, and what device would be sending jumbo packets to the hub?

There are a lot of absolute statements of "not needed" in this thread. Every use-case is different. Ultimately jumbo frames are about network utilization efficiency, and there are lots of reasons why they might be needed or useful, regardless of network topology.

Also, jumbo frames are a link-level thing, so any network peer whose link(s) negotiate support for jumbo frames might be sending them. Think of high-bandwidth devices, like data streaming (cameras, hi-def audio/video, etc).

You have to know which of your devices and network infrastructure aren't compatible with jumbo frames and plan accordingly. HE isn't, so the suggestions here are correct to be defensive with network settings and device settings that may interact with HE in any way.

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Yes, they can. It happens with mDNS if you have enough services in your network.

And yes, it will crash your hub.

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a switch that doesnt support jumbo frames is supposed to split up the packets.. if you hub is behind on of those you should be fine.

For jumbo frames to work all the devices in the segment must be able to receive and transmit up to 9000 bytes. If lets say jumbo frames is set on the switch and the end unit does not accept it may decrease performance due to frames being dropped or fragmented. Also with jumbo frames is that even with jumbo frames on a compatible segment can still increase packet loss. There is no real standard to JF so even devices that support JF can also fragment or drop packets from other devices with a different implementation. It's a crap shoot. Best just leave it all at mtu1500 and have a reliable network instead of trying to eek out some speed on a home network...

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UDP multicast can use jumbo frames. It is also possible to create a oversized icmp packet that’s sent to the broadcast address.

Just want to add - I don't know how difficult this is, but Hubitat should be patched to be resistant to jumbo frames. It will probably mean just patching the kernel module for the network interface. But if there are issues while saving the new module to disk, it would essentially brick the device.

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