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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. Hello again Florian.

    Yes, you are correct, I was describing the Layer 2 MTU. The IP MTU is an additional parameter that you can configure on an interface such that all outgoing IP packets (payload + header) will never be larger than this value. Anything larger will be fragmented before it is sent out of the interface.

    Note that this is a parameter that can only be implemented on a layer 3 interface, that is, on an interface that has an IP address. Any layer 2 interfaces will not have this capability.

    In Rene's example, for the purposes of the lesson, he set the Layer 2 MTU to something smaller than the IP MTU on the same interface. This means that no IP packets larger than the layer 2 MTU (which was set at 1460) would successfully be transmitted from this interface. You would never put such a configuration on a router's interface in a production network. The IP MTU must be =< the Layer 2 MTU in order for traffic to be forwarded.

    In the real world, the IP MTU parameter would be adjusted if you know that you have links downstream that have smaller Layer 2 MTUs so that no fragmentation will take place when enapsulating from layer 3 to layer 2 and that no frames will be dropped because of MTU restrictions.

    By adjusting both the IP MTU and the Layer 2 MTU at the appropriate interfaces on your network, you can ensure that no frames will be dropped because of MTU restrictions.

    I hope this has been helpful!


  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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