Apa fungsi mtu atau l2 mtu di mikrotik

Mikrotik Security For Beginner's #1 73 Perhitungan MTU pada MikroTik Maximum Transmission Unitadalah istilah dalam teknologi informasi yang merujuk kepada ukuran paket data terbesar yang dapat ditransmisikan melalui sebuah media jaringan. Ukuran MTU adalah bervariasi, tergantung teknologi jaringan yang digunakan. Contohnya adalah dalam jaringan berbasis teknologi Ethernet, ukuran MTU maksimum adalah 1500 bytes. Adalah tugas lapisan data-link yang harus menentukan ukuran MTU. Setting MTU biasanya dilakukan pada perangkat networking semacam switch, router dan sebagainya. Sangat jarang melakukan setting MTU secara manual pada workstation atau host. Jika ip layer menerima paket yang akan diteruskan ke dalam jaringan, maka perangkat akan mengkalkulasi ukuran paket jika ditambahkan dengan 20 bytes ip header. Jika ternyata paket yang akan dikirimkan memiliki ukran MTU yang lebih besar dari MTU perangkat yang menerima paket tersebut, maka paket akan difragmentasi, atau dipotong menjadi ukuran yang lebih kecil. Nilai MTU yang besar memungkinkan untuk mengirimkan data lebih cepat, bayangkan jika Anda punya banyak data kemudian dikumpulkan jadi satu box besar. Maka kita cukup mengirimkan semua data satu kali menggunakan satu box besar. Namun jika untukran box kecil, maka kita perlu mengirim beberapa kali. Nilai MTU besar tidak kemudian selalu berefek baik. Semakin panjang MTU, semakin tidak reliable proses pengiriman data. Jika ada kerusakan paket dalam pengiriman maka seluruh paket yang rusak akan dikirim ulang oleh protokol TCP (Transmission Control Protocol). Begitu juga pada sistem di MikroTik yaitu RouterOS yang mana memiliki ukuran standart dari besar MTU masing-masing interface. Pada RouterOS untuk MTU ini dibagi ke beberapa tipe yaitu L3 MTU(IP/Layer-3), L2.5 MTU(MPLS/Layer2.5), L2 MTU(MAC/Layer-2), Full Frame MTU. Dari tipe-tipe MTU tersebut memiliki nilai standart yang berbeda-beda.

Mikrotik Security For Beginner's #1 74 L3 MTU = 1500 (Data: 1480 + IP:20), L2.5 MTU = 1504 (Data: 1480 + IP: 20 + MPLS: 4) L2 MTU = 1508 (Data: 1480 + IP: 20 + MPLS: 4 + VLAN:4) FULL Frame MTU = 1522 (Data: 1480 + IP: 20 + MPLS: 4 + VLAN: 4 + ETH:14). Perbedaan besar nilai setiap tipe MTU tergantung pada jenis trafik yang akan di handle. Jika ukuran MTU hanya dipatok di nilai 1500 (L3 MTU) tanpa menambahkan tipe MTU Yang lain maka untuk trafik dengan service VLAN, MPLS, tidak akan bisa lewat. Pada kondisi normal paket data yang memiliki ukuran terlalu besar akan di fragment oleh perangkat secara otomatis. Terlalu banyak paket yang difragment akan mengakibatkan antrian paket yang juga panjang, kemudian perangkat yang menerima paket juga harus menyusun kembali paket yang diterima. Ukuran MTU yang disupport produk Mikrotik bisa dilihat disini. Terlebih jika ada aplikasi yang membutuhkan nilai paket data yang statis, maka admin jaringan harus mampu menentukan ukuran paket data yang akan dilewatkan agar paket dapat diterima dengan baik. Beberapa service yang mengirimkan data lewat jaringan biasanya akan menambahkan header pada paket data, misalnya ping.

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Topic Author

Sun Sep 22, 2019 12:23 am

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 ...

Forum Guru

Sun Sep 22, 2019 1:27 pm

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).

BR,
Metod

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Topic Author

Sun Sep 22, 2019 2:13 pm

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

Forum Guru

Sun Sep 22, 2019 3:01 pm

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.

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Topic Author

Sun Sep 22, 2019 6:28 pm

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.

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....

Forum Guru

Sun Sep 22, 2019 10:01 pm

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).

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Wed Sep 25, 2019 1:15 pm

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).