Count the border gateways a data centre needs based on throughput, NAT session scale and redundancy.
Gateways are limited by two independent budgets — packets per second expressed as throughput, and stateful session table size — and the binding constraint is whichever needs more boxes. Redundancy units are then added on top so a failure or upgrade does not reduce capacity below demand. Session tables usually fill before bandwidth does on NAT and firewall gateways, and this shows which of the two is driving the hardware count.
DC Gateway Capacity
active gateways = max(throughput ÷ per-gateway throughput, sessions ÷ per-gateway sessions), rounded up; deployed = active + redundant units.
active gateways = max(throughput ÷ per-gateway throughput, sessions ÷ per-gateway sessions), rounded up; deployed = active + redundant units. Gateways are limited by two independent budgets — packets per second expressed as throughput, and stateful session table size — and the binding constraint is whichever needs more boxes. Redundancy units are then added on top so a failure or upgrade does not reduce capacity below demand.
Session tables usually fill before bandwidth does on NAT and firewall gateways, and this shows which of the two is driving the hardware count.
This calculator takes 5 inputs: North-south throughput, Concurrent NAT or flow sessions, Throughput per gateway, Session capacity per gateway, Redundancy model. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.