Hi, I am bit confused with MTU and L2MTU value. I am vondering if it should be same on two connected device or better keep it as router set it up?
I am having L2 MTU on
RB600 - 1600
hAP ac2 - 1598
RB1100Hx4 - 1592
RB4011 - 1592
HeX PoE - 1598
I was reading about packet size but not sure if this value can be reason of smaller thoput?
Should I setup all davices on network to same L2 MTU (ethernet)? Or leave it on value set by router? Is it related to each router? Why its different …
L2 MTU is maximum (ethernet) packet size each of physical interfaces is capable of transmitting (and receiving).
MTU is maximum L3 (IP) packet size which can pass the interface and should be less or equal to the L2 MTU. At the same time it should be set to whatever size peers (accessible through that device) are capable of receiving (that’s true for both MTU sizes). In addition to that you have to make sure that non-routed packets can flow end-to-end meaning that it is advisable to have same MTU (the L3 one) on all switched/bridged interfaces.
It is possible to have different MTU on different L3 networks, the router can fragment packets if needed (when sender doesn’t set “dont fragment” flag on IP packet). This is not welcome though as final receiver then has to de-fragment packets, leading to additional delay and increased use of buffer memory by IP stack (but I still think that excessive use of DF flag these days is bad thing because it sometimes breaks connections instead of them being somehow slower).
Thank you for your post.
I was wondering why different Mikrotik device get “by default” different L2MTU …
IF I connect HeX PoE and RB4011 and each will have on connected ether ports different L2 MTU - 1592 and 1598, will it slow down dataflow - becouse one have to fragment packets? Should I set it up same on both sites? Or there is some HW reason, why RB4011 got on all ether ports by default L2 MTU 1592?
The “first MTU / i guess L3 MTU is always 1500”
RB4011 - 1592
HeX PoE - 1598
Different hardware has different L2MTU. However it is not important, unless you are doing MPLS and need a lot of L2MTU.
For normal use it is only MTU that matters.
I see, so i can not expect big change if i change it to same value as other clnnected Mikrotik devices? Its better keep it as it is from default for this particular Mikrotik router - RB4011 - 1592?
Just having wierd behiving. I got 500Mb Internet and when i connect laptop directly to modem it works. Yes I am using some internet speed tests (4 dfferent) but when i connect over Mikrotik first test go close to 500Mb and all others fall on half. CPU not get over 10% so MTU was one of the ideas, what can be wrong…
L2MTU has nothing to do with CPU usage. It is just a limit on the size of the low-level frames.
Maybe it has some impact on memory usage (size of buffer used when receiving frames).
I never touch it, I just leave it at its value decided by RouterOS.
When using link level protocols that add lots of headers it can be required to increase it (notably MPLS).
Thnx