MTU

[Catalin Sandu]

Hi all,

I have some doubts regarding MTU.

MTU is related to the hardware capabilities of an network interface let's say or is a value in the IP header ?

Thank you,

Catalin

[PaulP · Accepted solution]

Hi,

It's neither. MTU is a unit controlled in software by each device to set the (spoiler alert) maximum transmission unit. Some protocols may carry the MTU size in the header for other purposes but this is not a general rule. Also the MTU can in some cases depending on platform be increased to 9000 byes (jumbo frames) rather than the more common 1500 bytes.

A good read on this here:

https://supportforums.cisco.com/t5/other-network-infrastructure/why-the-mtu-size-is-1500/td-p/105418

Hope it helps,

Paul

[Julio E. Moisa]

Hi,

As Paul mentioned, MTU or Maximun Transmission Unit, is the largest size in Bytes supported by certain layer to be forwarded. The default Ethernet MTU is 1500 Bytes and it can be displayed executing show interface <interface ID>. In few words each Ethernet frame can carry up to 1500 Bytes. On the switches the payload is 1500 + 18 Bytes for header, obtaining 1518 Bytes and for dot1q it will be 1522 Bytes. 

In some situations is required Jumbo frames, it is bigger than the standard Ethernet frame, 9000+ Bytes. Not all the devices support Jumbo frames. 

[Catalin Sandu]

Thank you for your answers!

What I wanted to find out, is in what header field the MTU value is found ? Or in other way said MTU value is found under what header and what field ?

[Julio E. Moisa]

Hi

Please check this link:

https://networklessons.com/cisco/ccie-routing-switching/pppoe-mtu-troubleshooting-cisco-ios/

[Joseph W. Doherty · Accepted solution]

it's a hardware limitation as defined by the standards for a particular media.

Where it gets confusing, in something like Ethernet, later NIC hardware has been designed to support larger non-standard MTUs, generally known as jumbo Ethernet. Since this is non-standard, when used, you need to insure hardware on the same "wire" can support the same MTU.

Also where it gets confusing, software often allows you to set a MTU smaller than what the hardware is actually capable of. This might also be done by default, as it often is for jumbo capable Ethernet NICs.

IP also has its own maximum packet size, 64 KB. If an IP packet is larger than what the media MTU support, IP supports fragmenting its packets to fit within the media MTU, For multiple reasons, generally you want to avoid IP fragmentation, so often IP packet sizes are limited to the minimum MTU supported between a set of hosts, end-to-end. (If you're not already familiar with it, you might want to read about PMTUD.)

[Catalin Sandu]

thank you all for your helpful answers and clarifications!

[Cormac Champion]

In IOS, i notice there are two possible MTU commands - system mtu xxxx and also system mtu jumbo xxxx.

Are Jumbo frames recognised differently than regular frames and therefore get handled differently thereby allowing for both of the above commands to be used simultaneously ?