Hi,
I have a server on a datacenter which is going out of BW for this month , it is normally on a 100MBPS port , today I tried to change its nic speed to 10mbps using ethtool , it seemed to work fine then I changed it back too 100mbps again and after some seconds , it went offline !
here is what I did :

[root@rome ~]# ethtool -s eth0 speed 10 duplex full autoneg off
[root@rome ~]# ethtool eth0
Settings for eth0:
Supported ports: [ MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: Not reported
Advertised auto-negotiation: No
Speed: 10Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: off
Supports Wake-on: g
Wake-on: d
Current message level: 0x000000ff (255)
Link detected: yes
[root@rome ~]# ethtool -s eth0 speed 100 duplex full
[root@rome ~]# ethtool eth0
Settings for eth0:
Supported ports: [ MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: Not reported
Advertised auto-negotiation: No
Speed: 100Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: off
Supports Wake-on: g
Wake-on: d
Current message level: 0x000000ff (255)
Link detected: yes
[root@rome ~]#

why did my server go offline ? isnt it safe to change nic speed when it is under load ?
Cheers

Obviously not, but do you really need to change the speed while its active?

I didnt know it is not safe to do it , so how do you suggest me to change its speed ? what should I do exactly ?
Cheers

I would guess you would take the interface down before changing the speed?

may be for a second , but it didnt disconnect my SSH session when I changed it to 10MBPS , it was working fine , but it went offline after I changed it back to 100MBPS.