MTU Size

Now we understand the MTU, let’s see the possibility of increasing the L2 MTU which differs from one MikroTik Router (or Switch) to another. Here below you can see an illustration demonstrating this:

You can see clearly that each model can have a bigger or smaller L2 MTU. When you increase the L2 MTU to a bigger number, this will be called a Jumbo frame.

Jumbo frames are frames which have a bigger MTU than 1500 byte. Normally we do not see this in the ISP environment, but we may see it in the data centers environment or server farms where they require to have a big MTU to allow faster communication on layer 2 networks. Think of something like universities or hospitals which have a lot of servers and want the traffic to flow faster on layer 2, that’s where you can use Jumbo Frames. In this case, all they need to do is to increase the Layer 2 MTU to the size that they want and what the router /switch allows them to increase it to.
Always remember that the Jumbo frames are inside your network, once you need to go to the internet, and then you require the MTU to be 1500 bytes.

Let’s see now in a normal production network how the MTU works. Let’s start with the Layer 3 MTU:

You can see that the packet which came inside the router had a layer 3 MTU of 1500 bytes and when it left the router it also had a Layer3 MTU of 1500 bytes. So, you see that the MTU is the same on Layer3 and it is highly required that you don’t change the MTU on Layer3 and keep it 1500 bytes.

Let’s check the MTU on Layer 2 now:

As you can see, the frame came in with an L2 MTU of 1500 bytes. The router has added a VLAN tag to the frame then the L2 MTU when leaving the router, it has increased its size and became 1504 bytes. In case you are using Q-in-Q (will speak about it later in this course) then it will carry more VLAN’s, which makes the L2 MTU go even higher. That’s why MikroTik is making the default L2 MTU as 1592 bytes thinking that you may not exceed this level when having Q-in-Q as an example.

To mention that it is very important when you decide to increase the MTU to know what you are doing otherwise in case this is done incorrectly then you fall into an issue with Fragmentation which means that the router will fragment the big MTU’s packet/frame which would cause a delay in sending the data and cause the router to use more his resources such as his CPU.


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