PoE Budget and Cognitive Campus Access Switch Selection
A new building needs 96 access ports. Planned devices: 40 PoE IP phones (Class 4, ~15.4W each), 20 PoE access points (Class 6, ~51W each), 10 PoE cameras (Class 4, ~12.9W each), and 26 non-PoE desktops. The team proposes two 48-port Cognitive Campus PoE switches, each with a 740W PoE budget, dual-homed via MLAG to the building distribution pair. Uplinks are 2x10G per switch. The building has a single power feed per IDF. The team wants to know if the design meets power and resiliency requirements.
1. Calculate the PoE demand and determine whether two 740W switches suffice, including a per-switch distribution.
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Demand: phones 40 x 15.4W = 616W; APs 20 x 51W = 1020W; cameras 10 x 12.9W = 129W. Total PoE = 1765W. Two switches at 740W each = 1480W aggregate budget, which is less than 1765W. Insufficient by 285W even with perfect balance.
Per-switch: 96 ports across two 48-port switches means 48 PoE-capable ports per switch if all PoE devices are spread evenly, but the 26 non-PoE desktops reduce PoE port count. Distribute the 70 PoE devices roughly evenly (~35 per switch): ~8 phones (123W) + ~10 APs (510W) + ~5 cameras (65W) = ~698W per switch, which is under 740W but with only ~42W headroom - too tight for inrush, future adds, or uneven distribution. A single AP-heavy switch could exceed 740W.
Judgement: two 740W switches are marginal-to-insufficient. Either select higher-budget PoE models, add a third switch to spread AP load, or reduce AP count per switch.
2. Assess the resiliency concerns given the single power feed per IDF and the MLAG uplinks.
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Single power feed per IDF: if that feed fails, both switches in the IDF lose power simultaneously, so MLAG dual-homing does not protect against IDF power loss - both MLAG peers are in the same failure domain. The MLAG uplinks protect against a single switch or single uplink failure, not against IDF power loss. This is a resiliency gap that must be documented and accepted or mitigated (e.g., dual feeds, UPS).
MLAG uplinks: 2x10G per switch is reasonable for 96 access ports, but verify the peer-link can carry transit traffic if one switch's uplink fails (see peer-link sizing). Also confirm PoE budget per switch after a peer failure - if one switch must power devices normally on the other, its budget will be exceeded.
Judgement: the design has acceptable switch-level redundancy but a single point of failure at the IDF power feed that must be explicitly accepted or mitigated.