09-20-2017 02:12 AM - edited 03-08-2019 12:06 PM
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
Solved! Go to Solution.
09-20-2017 03:50 AM
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
09-20-2017 05:55 AM
09-20-2017 03:50 AM
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
09-20-2017 04:12 AM - edited 09-20-2017 04:20 AM
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.
09-20-2017 04:19 AM
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 ?
09-20-2017 04:24 AM
Hi
Please check this link:
https://networklessons.com/cisco/ccie-routing-switching/pppoe-mtu-troubleshooting-cisco-ios/
09-20-2017 05:55 AM
09-27-2017 12:16 PM
thank you all for your helpful answers and clarifications!
11-29-2017 05:55 AM - edited 11-30-2017 07:18 AM
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 ?
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