For OSI model we focus on the bottom 4 layers: Transport: you need to know the difference between TCP and UDP, which applications uses TCP and which use UDP, how is TCP more reliable than UDP ( three-way handshake ). For L3 you need to focus on routing protocols ( EIGRP, OSPF mainly and few information about RIPV2) this wont be a deep dive into those protocols just general information. For L2 you need to explain what VLANs are and how do they work, most important L2 protocols being STP and . 1Q. you will also be asked about VTP and how it works. The OSI Questions are endless and the questions will vary depending on who is interviewing you and the team you are interviewing for. i. e firewall and VPN will focus on L4, enterprise teams will focus on L3 and L2.. etc