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Why would increasing the VLAN MTU size cause things to break?

We have a customer who wants 2000 byte packets for some reason, so I increased all the physical links to 9216, which didn't cause any problems, but it didn't actually allow the 2000 byte packets because you also have to increase the mtu on the vlans involved. On one of the intermediate routers (a Juniper EX4200), I increased the vlan mtu to 9150 (a convenient number smaller than 9216 for vlan overhead) and a bunch of other connections through the router broke. I could see making an MTU smaller breaking things, but not making it bigger...

abatie
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Why would increasing the VLAN MTU size cause things to break?

Not necessarily, but it might very easily. There's no negotiation for the MTU used on a VLAN - so any and all hosts, routers and switches on that VLAN need to support the configured size. Hosts and routers also need to be configured with the exact same MTU.

Any host that has its link MTU or frame size set to a larger value than the switch it's connected to (or any switch its frames pass through) sends out invalid frames and the switch is simply going to drop them as giants. The same goes for receiving hosts or routers.

In some situations you could get away with mixed MTU sizes but that's hard to maintain and not really practical.

If you configure a larger-than-standard frame size on the switches alone, that doesn't really hurt anything but doesn't achieve anything either.

Zac67
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  • For an intermediate device though, the packet size is going to be constrained by the neighbor devices - increasing an MTU in the middle shouldn't have any effect. – abatie Mar 04 '22 at 00:12