An MTU larger than the common ethernet standard of 1500 bytes is in use, and that MTU corresponds to a commonly used jumbo frame size.
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MTU, or maximum transmission unit, is also known as link MTU. Measured in bytes, it's the maximum packet size that can be sent in one piece over a hop without being fragmented. It differs from path MTU (PMTU) which is the minimum MTU of the hops in a path. In the presence of packet loss the path MTU discovery process can generate an underestimate for the MTU. One way to verify the discovered MTU is to re-run the test a couple times to see if the result is consistent. That said, the fact that the discovered MTU is associated with a known standard suggests that it is not an accidental underestimate.
Possible secondary messages
'Packet loss conditions may have resulted in an underestimate'
'4470 bytes corresponds to IP over SONET'
'9000 bytes corresponds to a common Ethernet jumbo frame size'
'9180 bytes corresponds to IP over ATM and a common Ethernet jumbo frame size'