ESXi MTU sizes?!

[SteveR123]

Hi folks, this will be a quick one and apologies if it has been asked before (no doubt). I've basically been using 1500 byte MTU size on all of my (VM) networks - vSwitches & dVSwitches. I've never heard of a requirement to change this (Jumbo frames aside). The links are all trunk links so the VLAN tagging is done on the ESXi side.

Today this article was brought to my attention , in particular the following:

microHOWTO: IEEE 802.1Q VLAN Tutorial

"

• To use the standard network-layer MTU of 1500 bytes, the equipment must support a link-layer MTU of at least 1522 bytes.
• If the link-layer MTU were limited to the standard value of 1518 bytes then the network-layer MTU would need to be reduced to 1496 bytes to compensate.

"

I'm no expert, but this is new to me. Have I been incorrect and should be using 1518?

Thanks in advance and kind regards

Steve

[MKguy · Accepted solution]

No, you don't need to change anything. MTU is typically referred as the maximum payload size of an ethernet frame without the layer 2 headers. The 802.1q VLAN header adds 2 byte and a prepended additional 2 byte protocol identifier is needed to signalize that this is a 802.1q tagged frame, thus increasing the maximum total frame size from 1518 to 1522, but not changing the actual size of the payload, aka MTU.

Any network device from the last decade capable of using VLANs should automatically allow for a maximum total frame size of 1522 bytes.