Hi @Dieter Ruhoff :
Check this guide:
https://www.cisco.com/c/dam/en/us/td/docs/switches/lan/csbms/550xseries/cli_guide/CLI_Tesla_Sx550X_SG550XG_2_2_5.pdf#page385
17.6 speed
To configure the speed of a given Ethernet interface when not using auto-negotiation, use the speed Interface (Ethernet, Port Channel) Configuration mode command. To restore the default configuration, use the no form of this command.
Syntax
speed {100 | 1000 | 10000}
no speed
Parameters
• •
100—Forces 100 Mbps operation
1000—Forces 1000 Mbps operation
10000—Forces 10000 Mbps operation Default Configuration
The port operates at its maximum speed capability.
Command Mode
Interface (Ethernet, Port Channel) Configuration mode
User Guidelines
The no speed command in a port-channel context returns each port in the port-channel to its maximum capability.
Example
The following example configures the speed of te1/0/4 to 100 Mbps operation. switchxxxxxx(config)# interface te1/0/4
switchxxxxxx(config-if)# speed 100
Regards
