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Forum Replies

  1. Hi Marian,

    It will depend on the router, some routers can’t set the “Eternet” MTU. IP MTU will always work.

    I changed the “ip mtu” to 1400 which will cause IP packets to be fragmented if they exceed 1400 bytes. If there is something in the path that doesn’t allow fragmented packets then those packets will be dropped. It’s better to adjust the TCP MSS as well, if you set it at 1360 then the total MTU is 1400 bytes (40 bytes for headers) which ensures that we don’t need fragmentation.

    Does that make sense?


  2. Hi Rene,

    As you mention the interface MTU for both end doesn’t need to be same.

    If let say R1 with interface MTU 9000 and R2 with interface MTU 1500. It will not cause any issue?

    Because the R1 sending 6 times more packet in one frame, R2 will able to receive it without any problem?


  3. Hello Florian.

    The IP MTU parameter is helpful when you know that the packets, as they move downstream from your router, will encounter Layer 3 MTUs smaller than the default 1500 bytes.

    (An example of such a situation is the traversal of a QinQ VLAN that requires an additional 4 bytes in the header of the Layer 2 frame which results in a maximum MTU of 1496.)

    Like I mentioned before, it is unlikely that you would configure a Layer 2 MTU parameter smaller than a Layer 3 MTU parameter on the same port. This would result in any IP packets larger than the Layer 2 MTU attempting to exit that port being blocked if the DF flag is 1 or being fragmented if the DF flag is 0.

    As for packets arriving at the router on that port, yes, the Layer 2 MTU would be checked.

    I hope this has been helpful!


  4. Hi Laz,

    thanks for your help!


  5. Hello AZM

    The MTU is checked on both inbound and outbound. So in your scenario, if the MTU on the outbound interface of Router 1 is 1500, then the frame of 9000 bytes that was sent and was successfully egressed through the inbound interface, will be fragmented. (unless the DF bit is set to 1 where it will be dropped).

    I hope this has been helpful!


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