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C6500 MTU Discrepancy between cards

Hi there,

I am testing an MPLS connection and found some funky discrepancy on the default behaviour of the cards. The connection was made between a C6506 and a Cisco 3945 running MPLS and OSPF

Basically what I have found is the following:

  • •1-      WS-X6148A-GE-45AF is able to fragment by default and manages frames up to 1508 without fragmenting even when the port shows MTU 1500
  • •2-      WS-X6748-GE-TX  doesn’t fragment by default and starts fragmenting once the MTU has been changed to 9216
  • •3-      WS-X6748-GE-TX  with increased MTU matches the default behaviour of a WS-X6148A-GE-45AF

I have read datasheets and most of the related documentation for both tipe of cards (both are running on CFC mode) and simply can't find a word explaining why it works like this, is it an architecture difference?

Please have a look at the attachment with the tests I have run so far

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