if mtu size is 1500 by default than why we can set upto 1600

[krahulbgp]

if mtu size is 1500 by default than why we can set upto 1600

[marce1000]

 - Strange question  , as the device shows a configurable mtu with default of 1500. The question would be more , why would you like to change the default value ?

 M.

[balaji.bandi]

The MTU was agreed upon between Device 1 (Source) to Devive 2 ( Destination)  - that means if they can support it, you can also increase in the modern device up to 9K Jumbo frames.

But changing the value of MTU may have other issues, so make sure you understand before changing.

[Giuseppe Larosa]

Hello @krahulbgp ,

the main reason to increase MTU is to avoid fragmentation or to accomodate MPLS traffic flows.

I would say it is the first case, if possible MTU is increased to avoid fragmentation if any form of overlay VPN ise used for example IPSec VPN.

if the router is a CE customer facing device on an MPLS L3 VPN or MPLS L2 service you don't need to change the MTU.

Hope to help

Giuseppe

[Flavio Miranda]

Hi

 "Set by default " does not means immutable . Default value is just a vendor recomendation for a initial  value. But the vendor can give you more possivilities , in case this default value, does not fit properly in your scenario. 

[MHM Cisco World]

you can config frame MTU with 1600 which called baby giant BUT not all SW support this size 
please find below link for baby giant.

https://www.cisco.com/c/en/us/support/docs/switches/catalyst-4000-series-switches/29805-175.html

[Joseph W. Doherty]

I would assume because that hardware supports an MTU larger than 1500, in this case, 1600.

Understand, there is no standard, for Ethernet, for MTU larger than 1500.  I.e. It's up to hardware capabilities.

That said, reasons for supporting a "baby giant" (the correct term for a 1600 byte Ethernet MTU, as noted by @MHM Cisco World) are noted by both MHM's reference and within @Giuseppe Larosa posting.

For another description of Ethernet giants, including "baby giants", you might also look at: this. 

[Richard Burts]

I like the approach suggested by @Flavio Miranda that the default is a value recommended by the vendor. It is intended to fit many cases but is changeable depending on local circumstances. I would be very surprised if the default was either the lowest or the highest value possible. From that suggested value that fits the majority of cases you might need to go smaller or to go larger.

[Jitendra Kumar]

See- this is the max MTU value that you can set manually.

default value does not mean the device had max as default.

It depends on the OEM and what they have standard device config settings.

deferent OEMS can set deferent MTU as well as per the policy.