Configuring a default stacking port to function as a data port
You can configure one of the two default stacking ports as a stacking port and the other port as a regular data port. By default, the 4-byte Ethernet preamble for the Ethernet frame is used when a port is configured as a default stacking port. The shorter preamble compensates for extra overhead required by the stacking protocol. To use a default stacking port as a regular data port, the Ethernet preamble must be set to 8 bytes.
To configure a default port to use the longer preamble, enter the long-preamble command at the interface level of the CLI.
device(config)# interface ethernet 1/2/1 device(config-if-e10000-1/2/1)# long-preamble
Syntax: [no] long-preamble
Use the no form of the command to revert to the 4-byte Ethernet preamble on a default stacking port.