Manual:Maximum Transmission Unit on RouterBoards

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Background

It is sole responsibility of administrator to configure MTUs such that intended services and applications can be successfully implemented in network. In other words - administrator must make sure that MTUs are configured in a way that packet sizes does not exceed the capabilities of network equipment.

There are several different values of standard (IEEE) MTU out there and it might cause some confusion:

  • Full frame MTU -1518 bytes - for standard (IEEE) Ethernet frames, this include Layer-2 header (14 bytes) and FCS (Frame Check Sum) 4bytes - this is MTU of the frames that travel thought the wires
  • Physical/ Layer-2 MTU1514 bytes - for standard (IEEE) Ethernet frames, this include Layer-2 header (14 bytes), but without FCS - This is MTU of the frame that software work with
  • IP/ layer-3 MTU1500 bytes - for standard (IEEE) Layer-3 (IP) packets, this excludes both Layer-2 header and FCS - This is value that can be configured in MikroTik RouterOS

So basically all those values are the exactly the same thing only from different point of view (from physical, Layer-2 and Layer-3 point of view)

Originally MTU was introduced because of the high error rates and low speed of communications. Fragmentation of the data stream gives ability to correct corruption errors only by resending corrupted fragment, not the whole stream. Also on low speed connections such as modems it can take too much time to send a big fragment, so in this case communication is possible only with smaller fragments.

But in present days we have much lower error rates and higher speed of communication, this opens a possibility to increase the value of MTU. By increasing value of MTU we will result in less protocol overhead and reduce CPU utilization mostly due to interrupt reduction.

This way some non-standard frames started to emerge:

  • Giant or Jumbo frames - frames that are bigger than standard (IEEE) Ethernet MTU
  • Baby Giant or Baby Jumbo frames - frames that are just slightly bigger that standard (IEEE) Ethernet MTU

It is common for ethernet interfaces to support physical MTU above standard, but this can not be taken for granted. Abilities of other network equipment must be taken into account as well - for example, if 2 routers with Ethernet interfaces supporting physical MTU 1526 are connected through Ethernet switch, in order to successfully implement some application that will produce this big Ethernet frames, switch must also support forwarding such frames.

MTU on RouterBoards

Here is a table of MTU (with Layer 2 header, but without FCS) supported by Mikrotik RouterBoards:

RouterBoard ether1 ether2 ether3 ether4 ether5 ether6 ether7 ether8 ether9
RB493, RB493AH 1522 (+8) 1518 (+4) 1518 (+4) 1518 (+4) 1518 (+4) 1518 (+4) 1518 (+4) 1518 (+4) 1518 (+4)
RB450 1522 (+8) 1518 (+4) 1518 (+4) 1518 (+4) 1518 (+4)
RB433, RB433AH 1522 (+8) 1518 (+4) 1518 (+4)
RB411, RB411A, RB411AH 1522 (+8)
CrossRoads 1600 (+86)
RB1000 9500 (+7986) 9500 (+7986) 9500 (+7986) 9500 (+7986)
RB600, RB600A 9500 (+7986) 9500 (+7986) 9000 (+7486)

RouterOS features and MTU

There are several features in MikroTik RouterOS that is can benefit from possibility to exceed standard MTU

VLAN

When tagged by VLAN-id (802.1Q) inserts a 4-byte tag and recomputes the frame check sequence (FCS): requires additional 4-bytes

MPLS

PPPoE

EOIP

PPTP