DC Gateway Capacity Calculator
Count the border gateways a data centre needs based on throughput, NAT session scale and redundancy.
Inputs
Gateways to Deploy
4units
Active Gateways Required
3units
Utilisation of the Active Set
83.33%
Spare Throughput
120.0Gbps
Gateways Needed for Sessions Alone
1units
Step by step
Values used
North-south throughput = 200 Gbps; Concurrent NAT or flow sessions = 2,000,000 sessions; Throughput per gateway = 80 Gbps; Session capacity per gateway = 4,000,000 sessions; Redundancy model = N+1
DC Gateway Capacity
active gateways = max(throughput ÷ per-gateway throughput, sessions ÷ per-gateway sessions), rounded up; deployed = active + redundant units.
Gateways to Deploy
= 4 units
Active Gateways Required
= 3 units
Utilisation of the Active Set
= 83.33
Spare Throughput
= 120.0 Gbps
Gateways Needed for Sessions Alone
= 1 units
How it works
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.
Formula
DC Gateway Capacity
active gateways = max(throughput ÷ per-gateway throughput, sessions ÷ per-gateway sessions), rounded up; deployed = active + redundant units.
- N+1
- One spare unit beyond the active set
- sessions
- Concurrent stateful flows the gateway must track
Frequently Asked Questions
How is DC Gateway Capacity calculated?
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.
Why does DC Gateway Capacity matter?
Session tables usually fill before bandwidth does on NAT and firewall gateways, and this shows which of the two is driving the hardware count.
What values do I need to enter?
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.