Are you sure its a gigabit adapter?
Try a lscfg -vl entN

You should see something like:

This is for a Gigabit adapter.

$ lscfg -vl ent2
 ent2              30-68            Gigabit Ethernet-SX PCI Adapter
(14100401)
        Network Address.............0002559A2F2F
        Displayable Message.........Gigabit Ethernet-SX PCI Adapter
(14100401)
        EC Level....................E78971
        Part Number.................41L6396
        FRU Number..................07L8918
        Device Specific.(YL)........P2-I7/E1

And you can try this to see the possible values.

This is for a 10/100 adapter.

$ lsattr -R -l ent0 -a media_speed

10_Half_Duplex
10_Full_Duplex
100_Half_Duplex
100_Full_Duplex
Auto_Negotiation

> -----Original Message-----
> From: Loon, E.J. van - SPLXM [mailto:Eric-van.Loon AT KLM DOT COM]
> Sent: Friday, February 22, 2002 6:51 AM
> To: ADSM-L AT VM.MARIST DOT EDU
> Subject: 1 Gb. Ethernet adapter
>
>
> Hi *SM-ers!
> We have a H70 server with a 1 Gb. Ethernet adapter. I checked the
> setting and it's set to Auto_Negotiation. I know that this is not the
> correct value for optimal performance, but I see only the following
choices:
> 10_Half_Duplex
> 10_Full_Duplex
> 100_Half_Duplex
> 100_Full_Duplex
> Auto_Negotiation
> There are no 1000_Half_Duplex and 1000_Full_Duplex options available.
> What should select here?