Infrastructure
Infrastructure questions cover the devices, segmentation, and connectivity that make a network work: routers, switches, access points, firewalls, VLANs, trunking, Spanning Tree Protocol, WAN links, NAT, and cloud models. You should be able to explain what each device does and how traffic moves between segments.
Core devices
A router connects different networks and makes routing decisions. A Layer 2 switch forwards frames using a MAC address table and supports VLANs. A multilayer switch can route between VLANs. An access point bridges wireless clients to the wired LAN. A firewall filters traffic by policy. A wireless LAN controller manages multiple access points. DHCP and DNS servers provide addressing and name resolution.
VLANs and trunks
An access port carries one VLAN. A trunk carries multiple VLANs using 802.1Q tags; the native VLAN is sent untagged. Inter-VLAN routing requires a router-on-a-stick with subinterfaces or a Layer 3 switch. Spanning Tree Protocol prevents Layer 2 loops by electing a root bridge with the lowest bridge ID and placing redundant ports in blocking state.
WAN and cloud
WAN options include leased lines, MPLS, broadband, and VPNs. NAT translates private addresses to public addresses. Cloud service models are IaaS, PaaS, and SaaS; deployment models are public, private, and hybrid. Data centers often use spine-leaf topologies for predictable east-west traffic.
Worked example
A 24-port switch carries VLAN 10 and VLAN 20. Ports 1-12 are access ports in VLAN 10; ports 13-23 are access ports in VLAN 20; port 24 is a trunk to a router. The router uses subinterfaces with 802.1Q encapsulation and an IP address in each subnet as the default gateway. Without the trunk and subinterfaces, hosts in VLAN 10 cannot reach hosts in VLAN 20 even though both VLANs share the same switch.
Traps
A trunk port is not an access port. A native VLAN mismatch can cause untagged traffic problems. STP blocking does not mean the physical link is down. A router interface needs an IP address in the subnet to route for it. Wireless clients mapped to different VLANs need the correct SSID-to-VLAN configuration at the AP or controller.