Cisco's 3850 series 24 and 48 port switches come with one expansion slot, allowing for the addition of up to 4 1GE or 10GE SFP ports to the device. By default, 3850 switches ship with no module installed, so any additional uplink ports must be purchased separately.
The C3850-NM-2-10G network module is compatible with all Cisco 3850 series switches, and provides 4 additional SFP/SFP+ interfaces to the device. These 4 ports can be configured as 4 individual 1 GE interfaces, or as 2 10GE interfaces. Interfaces not used for 10GE connections can still be used as gigabit Ethernet ports, allowing for all 4 ports to be used simultaneously, regardless if 0, 1, or 2 ports are configured as 10GE ports.
When deploying the C3850-NM-2-10G, be sure to use ports 2 & 3 for any 10GE connections, as ports 1 & 2 are only 1GE capable, as shown below:
The C3850-NM-2-10G is hot swappable, and can be replaced with either a C3850-NM-4-1G or a C3850-NM-4-10G should the need arise, however it should be noted that the C3850-NM-4-10G can only provide more than 2 10G ports on 48 port 3850 switches.