Big ip LTM - MTU size

What makes you think it's the MTU? Do you have a tcpdump showing that path-mtu discovery is failing somewhere?

Generally when a bulk transfer like a backup fail for MTU, it's when the backup first starts streaming.

Note that MTU's MUST be the same across all hosts on a VLAN. (Yes, pedantic, and you can get away with odd hosts having different MTU's because most stacks will receive packages larger than their configured MTU. Right up until something strange goes wrong, so best to adhere to the specs)

H