Verify that the speed and duplex settings are the same on both sides of the link.
If both sides are set to auto and problems persist, eliminate auto-negotiation issues by hard setting the speed and duplex on both sides.
Do not set one side to auto and hard set the other side, as this can result in a mismatch. To set the duplex and speed on a set-based switch, issue the set port speed and set port duplex commands.
For example, to set the speed to 100 and the duplex to full on port 3/2, issue the set port speed 3/2 100 and set port duplex 3/2 full commands.
On an Cisco IOS Software based switch, issue the duplex full and speed 100 interface configuration commands.
Verify that the flow control settings are the same on both sides of a Gigabit Ethernet link. The Gigabit auto-negotiation negotiates flow control, duplex mode and remote fault information. You must either enable or disable link negotiation on both ends of the link. Both ends of the link must be set to the same value or the link will not connect. The set-based command to control this setting is set port negotiation mod#/port# enable or disable.
The Cisco IOS Software based command is negotiation auto or no negotiation auto.