Design the wireless network infrastructure
Domain 2: Design the wireless network infrastructure
This domain tests the core RF and network-services design: AP placement, antennas, controller sizing, PoE, DHCP/DNS, multicast/mDNS, QoS, IPv6, and mesh/outdoor considerations. The exam expects you to justify a design with numbers.
RF and AP placement
Design to both coverage and capacity. For coverage, target about -67 dBm RSSI and 20 dB SNR at the cell edge, with 15-20% overlap between cells for roaming. For capacity, use 5 GHz as the primary band: in the US, 5 GHz offers up to 25 non-overlapping 20 MHz channels depending on regulatory domain, while 2.4 GHz offers only three (1, 6, 11). Use 2.4 GHz for legacy coverage, not capacity. Choose antennas by ceiling height and coverage shape: omnidirectional dipoles for open offices, patch or directional antennas for long corridors or high ceilings, and downtilt to avoid overshoot. Avoid placing APs directly above metal or in elevator shafts.
Controller and network services
Size the controller for APs, clients, throughput, and features. Catalyst 9800 models differ in AP and client scale, so verify the model rating against the design plus headroom. Provide DHCP with enough scope for peak clients and option 43 or DNS for AP discovery; use DHCP relay on the client VLAN. Design DNS for CAPWAP name resolution and for guest or portal names. For multicast applications, enable multicast on the WLC and design PIM and IGMP snooping in the wired network. For Bonjour/mDNS, use an mDNS gateway or service reflection so Apple services cross VLANs. Apply QoS end to end: trust DSCP on the switch, map to WMM access categories, use platinum for voice, and enable call admission control. Plan IPv6 dual stack if required.
Worked example
A 90,000 sq ft office has 900 users and 2,500 devices. Coverage at -67 dBm needs 80 APs, but capacity at 25 clients per AP needs 100 APs, so use 100. Each Wi-Fi 6 AP draws 30 W: 100 x 30 W = 3,000 W. A 48-port switch with a 740 W PoE budget powers only 24 APs at 30 W, so you need five switches (100 / 24 = 4.2, round up). Add UPS capacity for the switches and controllers, and size DHCP for 2,500 leases plus growth.
Traps
- Designing only for coverage and ignoring client density.
- Assuming 2.4 GHz three-channel reuse is sufficient for capacity.
- Forgetting option 43 or DNS and DHCP relay.
- Enabling multicast without wired PIM and IGMP design.
- Ignoring mDNS/Bonjour for Apple-centric environments.
- Under-budgeting PoE and UPS.