Forum Discussion
DennyK_144735
Nimbostratus
NimbostratusBigIP LTM port translation - port selection process
Hey folks,
We have a load-balanced FTP server system that formerly resided behind an F5 3400 running BigIP LTM 10.0.1. We have recently moved it to a new F5, a 3900 running version 10.2.4. On bo...
DennyK_144735Nimbostratus
NimbostratusWell, we have found a workaround that resolves this issue; when we set the Source Port setting to "Change" instead of "Preserve", the translated ports it assigns are chosen sequentially instead of randomly, which resolves the issue with ports being re-used too quickly. Don't know why the assigned ports are random with "Preserve" but sequential with "Change", but it does fix the problem.
daboochmeister
Cirrus
CirrusWhen you set source port to "preserve" or "preserve strict", the incoming port used by the client accessing will be used. That port is typically chosen at random by the network stack on the client; so the apparent randomness you're seeing on the F5 is a result of the F5 simply using that randomly-selected port. And if you have multiple clients incoming, it is possible to get conflicts (unless you e.g. set up a large SNAT pool).