To clear the DF bit or not to... Your thoughts?

[Ricky S]

Hey everyone,

Is it more beneficial to clear the DF bit at the router and allow fragmentation of an IP datagram or not to clear it and let PMTUD do what it does best? I have asked this question to my fellow network pros and everyone seem to have mixed thoughts. One said, "Clearing the DF bit fixed a lot of issues I had with applications sending data packets with the DF bit set which get dropped by the router causing the host to re-transmit the packet with lower MTU setting."

Other said, "PMTUD saves my day every day."

I want to know what are your thoughts and how do you implement this on your network?

[Joseph W. Doherty]

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Posting

The major pros and cons are, if your router needs to fragment, it make take a noticeable performance hit but PMTUD ICMP packets are sometimes blocked.  Normally I lean toward adhering to standards, i.e. if the packet indicates DF, then don't fragment the packet but try to inform the source.