Calculation of MTU on MikroTik

Maximum Transmission Unit is a term used in information technology, which refers to the size of the largest packet data that can be transmitted over a network media. MTU size is vary, depending on network technology used. An example is in the network based on Ethernet technology, the maximum MTU size is 1500 bytes. Is the data link layer of the task should specify the MTU size.

Setting the MTU is usually done on the kind of networking switches, routers and so on. Very rarely do the MTU settings manually on the workstation or host. If the ip layer accepts packets will be forwarded to the network, then the device will calculate the size of the package if added with 20 bytes ip header. If it turns out that the package will be delivered has ukran MTU larger than the MTU of the device that receives the package, then the package will be difragmentasi, or cut into smaller sizes.
 A large MTU value makes it possible to send data faster, imagine if you have a lot of data collected so one big box. Then we simply sends all the data one time using one big box. However, if the untukran a small box, then we need to send a few times. Large MTU values do not always effect conferring good. Getting longer, the MTU is not reliable data delivery process. If there is any damage to the package in the delivery of all the damaged package will be sent by the Protocol TCP (Transmission Control Protocol).
So too on the system MikroTik RouterOS i.e. which has a size standard large MTU of each interface. On RouterOS for MTU is divided into several types, namely L3 MTU (IP/Layer-3), L 2.5 MTU (MPLS/Layer 2.5), L2 MTU (MAC/Layer-2), Full Frame MTU. Type-type of the MTU value is standard.
L3 MTU = 1500 (Data: 1480 + IP: 20),
L 2.5 MTU = 1504 (Data: IP: 20 + 1480 + MPLS: 4)
L2 MTU = 1508 (Data: 1480 + IP: MPLS: 20 + 4 + VLAN: 4)
Full Frame MTU = 1522 (Data: 1480 + IP: MPLS: 20 + 4 + VLAN: 4 + ETH: 14).

The big difference of the value of each MTU type depends on the type of traffic that will be on the handle. If the MTU size is only pegged at a value of 1500 (L3 MTU) without adding The MTU type other then for the traffic with service VLAN, MPLS, will not be allowed to pass. In normal conditions the data package has a size too big will in fragment by the device automatically. Too many packages that difragment would result in long queues the packet as well, then the device that receives the packages also have to reconstitute packets that are received.
What if there are applications that require the data package value is static, then the network admin should be able to determine the size of the data packets will be passed so that packets can be received well. Some service that sends data over the network will usually add a header in data packets, for example ping.
At the time of data out of your device, data packet will be coupled with a couple of headers, IP Header among others 20bytes, and ICMP headers 8bytes. To do the test, you could use the command ping-f-s on windows OS, ping-M-s on Linux, or ping-D-s on Mac OSX. For example we try to ping with packet size 1500, without fragmentation.

 
That happened the other device can not receive because the packet size is too large. Why can such a device, though opponents have 1500 MTU settings?. The size of the data packet size is 1500 before added to the header. Then we can calculate the size of a ping package appropriate data can be accepted.
1500 = 20 (IP header) + 8 (ICMP header) + Data
Data = 1500-20 (IP header)-8 (ICMP header)
Data = 1472
If we try to ping with size 1472, then packages can be well received

Other service usually also added a header with a specific value, for example a VPN PPTP and PPPOE. This will add the pptp service header and checksum, whereas the PPPoE PPPoE PPP header, add ID, DST & SRC Address.
PPTP:
Data = 1500 – 20 (IP Header) – 28 (PPTP Header) – 2 (Checksum) = 1450
PPPOE
1500 – 6 (PPPoE Header) – 2 (PPP ID) – 12 (DST & SRC show addresses) = 1480
Calculation as above also apply when using the service lainBisa concluded that with increasing the size of the ip header, then the size of the data on which ditrasmisikan is also getting smaller. Miminum size MTU is 576 bytes.
Path MTU Discovery
To find out the value of your opponent's device MTU, a device that is connected to the network has a mechanism called with path MTU discovery. This mechanism is not at all membuthkan features or special service, but uses a fairly simple way i.e. by utilizing the mechanism of error reporting on ICMP.