Jumbo frames on switches

[hmc25000]

Can any issues arise when setting the MTU size to 9578 in Switch settings for the entire network consisting of all Meraki Switches?

I see in red in switch settings "MS425 switches: xxxx are limited by hardware to 9416".

We do have a couple of MS425 switches.

Thanks.

[cmr]

If you have MS425s then any traffic traversing them is limited to 9416 byte frames so if you set the network to 9578 byte frames then large frames between 9417 and 9578 bytes will be fragmented and that will cause issues.

[hmc25000]

Thanks. Can I assume backup file transfers and Video/audio traffic such as online meetings will likely be effected by this?

Would it make more sense to set the MTU to 9416 across the network in this case?

And what if the MTU size on the ISP router is still 1500? 

[cmr]

Jumbo frames will only be used if the endpoint is configured to use them.  the network setting is a maximum size and should be set to no more than 9416 in your case.  If I look at my laptop I see the below dialogue box for the ethernet adapter:

As you can see, jumbo fames are disabled so the network max size being above will be ignored and the default MTU of 1500 will likely be set.

At a command prompt type: netsh interface ipv4 show subinterfaces

This will confirm what is set for you.

[hmc25000]

Thanks again. On Netapps storage devices jumbo frames are likely used. How will fragmentation be effected if the WAN routers have a standard MTU size of 1500 with large amounts of traffic going over the WAN?

[cmr]

@hmc25000 in that case, if jumbo frames are sent from one site's NetApp to the other, the frames will either be fragmented (slowing the transfer) or dropped.  We actually changed all of our iSCSI SANs to not use jumbo frames as the performance benefit is often small, while the risk in anything other than an isolated network, is somewhat larger...

[RaphaelL]

If you can get a PMTU that supports Jumbo frame go for it. If you can't , stay away from it. Fragmentation and IP reassembly will be a performance nightmare as mentionned by cmr.

[Brash]

I'm with @cmr  . In an enterprise environment, while Jumbo frames locally for storage makes sense, pumping them over a WAN often doesn't purely due to the increased network complexity and risk factors.

[hmc25000]

So but then why would MEraki set jumbo frames as default MTU setting in Switch settings? At least that's what I see in my Organization, I have no reason to believe that someone changed it.

[cmr]

For local iSCSI.  If you have a SAN and a few local servers then jumbo frames are good.  Otherwise just don't use them.  

Having the switch support a large MTU, doesn't mean you have to use it!

[PhilipDAth]

Personally, unless you are a service provider (and if you are, why are you using Meraki), I don't see much advantage to using an MTU larger than 9,000 bytes.  So I would tend to use that as your maximum.  It is a more used safer number.

The only thing I would use jumbo frames for is IP-based storage.  And then only over the local LAN.