How to solve the wrong duplex setting of Cisco Router 3900

We have a Cisco 3900, and our ISP told us that our config (below) has the wrong duplex setting. They want us to change it to 100 FULL.

1. What is the command to change the duplex to FULL (without restarting the router).

2. What if the link goes down, how do I revert back to the old settings (AUTO)?

interface GigabitEthernet0/0

 description Internet via $ETH-WAN$$FW_OUTSIDE$

 ip address 209.xx.xx.xx 255.255.255.252

 ip access-group 101 in

 no ip redirects

 no ip proxy-arp

 ip verify unicast reverse-path

 ip flow ingress

 duplex auto

 speed 100

 no mop enabled

The solution:

try

interface GigabitEthernet0/0

spped 100

duplex full

sh int gi 0/0  will show the current settings .

or just try:

show int --> shows half duplex.. did u apply the commands

regarding question number 2 ...  no need to restart .

just change the duplex to  auto ( duplex auto)

or

int gi 0/0

shut

no shut.

The other end could also just not be Cisco 3900 series router, sometimes disparate hardware requires that you force the GB connection to Full.

The standard is to set 1Gbps connections to auto, but when it first came out we often had to force it to get it to work with some hardware.

Sometimes when the duplex negotiation fails it will fall-back to half-duplex.  Therefore it's not correct to say that the other end is definitely set to half.  Negotiation could fail for a number of reasons, including manually specifying the speed.

The main reason ISPs specify manual speed and duplex settings is because they want to mitigate any chance of auto-negotiation failure which would result in half-duplex operation and therefore reduce support incidents.