Auto-negotiation is a requirement for using 1000BASE-T according to IEEE standard.  Gigabit Ethernet needs link auto-negotiated to determine master and slave association pair.  Without auto-negotiation, the link clock will not form properly and underlying physical problem can occur.  Due to this reason, Packetshaper is following IEEE standard to ensure properly Gigabit Ethernet connections.  Following are extract from IEEE standard on 1000BASE-T.

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All 1000 BASE-T PHYs shall provide sup­port for Auto-Negotiation
(Clause 28) and shall be cap­able of oper­at­ing as MASTER or SLAVE.
Auto-Negotiation is per­formed as part of the ini­tial set-up of the link, and
allows the PHYs at each end to advert­ise their cap­ab­il­it­ies (speed, PHY
type, half or full duplex) and to auto­mat­ic­ally select the oper­at­ing mode
for com­mu­nic­a­tion on the link. Auto-negotiation sig­nal­ing is used for the
fol­low­ing two primary pur­poses for 1000BASE-T:

a) To nego­ti­ate that the PHY is cap­able of sup­port­ing 1000BASE-T half
duplex or full duplex transmission.

b) To determ­ine the MASTER-SLAVE rela­tion­ship between the PHYs at
each end of the link. 1000BASE-T MASTER PHY is clocked from a local source.
The SLAVE PHY uses loop tim­ing where the clock is recovered from the received data stream.

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Some vendors are providing the option of manual setting the speed of gigabit interface.  Since this is not defined in the IEEE 802.3ab standard, it is only recommended to manually set the interfaces when connecting to the same vendor products to prevent vendor incompatibility issues.