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I'm having trouble understanding the functional difference between an Up MEP and a Down MEP in CFM applications.

Some sites say they are defined based on network direction, others seem to think they are defined based whether the "other end" is a higher or lower level (which also doesn't make sense, but that's another topic).

Can anyone give a simple explanation of the difference between a Down MEP and an Up MEP, and when you would use which?

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Lets say you have a physical interface, if you send a cfm without proccessing it threw your devices logics then it is a down mp (from the port to the out side), if you process it threw your devices logic than it an up mep(from the port into your device and to the other MP's).

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Read this detailed note: http://www.cisco.com/c/en/us/support/docs/asynchronous-transfer-mode-atm/operation-administration-maintenance-oam/117457-technote-cfm-00.html

UP MEP:

  • CFM PDUs generated by the MEP are sent towards the Bridge's Relay Function and not via the wire connected to the port where the MEP is configured
  • CFM PDUs to be responded by the MEP are expected to arrive via the Bridge's Relay Function

Down MEP:

  • CFM PDUs generated by the MEP are sent via the wire connected to the port where the MEP is configured
  • CFM PDUs to be responded by the MEP are expected to arrive via the wire connected to the port where the MEP is configured

Up MEP is created on UNI where as Down MEP is created on NNI. Up Mep packets will follow service path and be transmitted out. Down Mep packets will be transmitted directly since it is created on NNI.

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