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Forum Replies

  1. Hi Rene,

    i have 2 questions :slight_smile: :

    1. When I try to set "mtu 1400" on the roter interface, I get the error: "% Interface FastEthernet0/0 does not support user settable mtu." What can be done in order to solve this error?
      1. In the first solution presented above in order to solve the MTU issue, why is not enough only the command "ip mtu 1400", since you have also added "ip tcp adjust-mss 1360"? Actually, wouldn't be enough only one of them, why have you used both?

    Thank you,

  2. 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?


  3. Hi Rene,
    Maybe I misunderstood this part of the paragraph
    "This means that a single Ethernet frame can carry up to 1500 bytes of data (without headers). Typical header sizes are 14 bytes for Ethernet (add another 4 bytes if you use 802.1Q tagging) and 20 bytes for an IP header"

    That is my source of confusion. You state than en ethernet frame can carry up to 1500 bytes of data (without headers). But MTU does include the IP header and TCP header (if tcp is in use).

    Maybe I am overthinking this :slight_smile:

  4. 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!


  5. Hi Laz,

    thanks for your help!


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