Ethernet - Maximum MTU

Unanswered Question
Apr 30th, 2009


Cisco 3750 switch is operating in pure L2 mode and the MTU is globally set to 1500bytes - tested successfully. A frame with a UDP packet is placed on the wire by the host and the size of the packet is 2048bytes. The df bit is set to '1' in the IPheader but the switch is operating in L2 mode so it will not inspect that. So, will the switch discard the Ethernetframe and IPpacket given that the maximum MTU size supported by the receiving port on the switch has been exceeded?

And if indeed it i.e switch drop these unsupported packets sizes I was expecting to see this captured with 'show controllers ethernet-controllers', but guess what?, nothing - counter reads 0

thank you


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Giuseppe Larosa Thu, 04/30/2009 - 10:08

Hello Ajaz,

it should count as a giant on receiving interface and dropped there

you should see the giant counter to increase by 1 at each attempt.

sh int type x/y

Unless the host fragments the UDP PDU regardless of DF settings

Hope to help


AJAZ NAWAZ Thu, 04/30/2009 - 11:00

giants is zero

drops is zero

MTU exceeded is zero

If we monitor the port it's not much help because we only see packets which make it through the buffer switch fabric.



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