Networking Stack- Layer-2(xSTP, LLDP, LACP etc) , Layer-3 (BGP, OSPF, PIM etc) , IPSec, IKE, GRE, EVPN/VxLAN, MPLS, QoS
Management -SNMP, Netconf/Yang, REST, CLI, gNMI, gRPC
System Features - Platform Bringup (Including implementing linux Device drivers), Stacking, High Availability etc.
Programming Language - C, C++, Python, GoLang languages
NOS - Experience in NOS development/sustenance.