The Universal Ethernet ports, or stacking ports, are located on the front panel of the switch and are labeled U1 and U2. These ports can be used with Switch Engine as stacking ports, or as Ethernet ports with Switch Engine and Fabric Engine in non-fabric mode (Fabric Engine does not support stacking).
The Universal Ethernet ports operate as stacking ports by default. Use the disable stacking-support command to set the U1 and U2 ports in Ethernet mode.
When used as Ethernet ports, the U1 and U2 ports support data rates of 100Gbs, 2 x 50Gbs, 4 x 25Gbs, or 4 x 10Gbs using QSFP28 optics. 4120 Series also has four SFP uplink ports that can be used as Ethernet ports that support data rates of 1Gbps, 10Gbps, or 25Gbps.
For information on switch stacking, see Build Stacks.