Manual:Maximum Transmission Unit on RouterBoards

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Revision as of 13:11, 19 December 2008 by Megis (talk | contribs) (MTU on RouterBoards)
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Background

There are several different values of MTU out there causing some confusion:

  • 1518 bytes - for standard (IEEE) Ethernet frame this include Layer 2 header (14 bytes) and FCS (Frame Check Sum) 4bytes - this is MTU of the frames that travel thought the wires
  • 1514 bytes - for standard (IEEE) Ethernet frame this include Layer 2 header (14 bytes), but without FCS - This is MTU of the frame that software work with
  • 1500 bytes - for standard (IEEE)Layer 3 (IP) packets (standard Ethernet frame without Layer 2 header and without FCS - This is value that can be configured in MikroTik RouterOS

So basically all those values are the exactly the same thing only taken in different places.

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

In order to communicate with such frames it is required that every device in the frame's way (even switches) support that particular frame size. As this is non-standard feature each vendor have different MTU supported.

MTU on RouterBoards

RouterOS scripting language has following data types:

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 1518 1518 1518 1518
RB433, RB433AH 1522 1518 1518
RB411, RB411A, RB411AH 1522
CrossRoads 1600
RB1000 9500 9500 9500 9500
RB600, RB600A 9500 9500 9000

RouterOS features and MTU

VLAN

MPLS

EOIP

PPTP