ARP Table Calculator
Size an ARP cache across VLANs and estimate the broadcast load its refresh cycle puts on every host.
About ARP Table Calculator
Every host in a VLAN must process every ARP broadcast in that VLAN, so the load each host sees depends on the size of its own broadcast domain rather than the whole device. The refresh rate is set by the aging timeout — a shorter timeout means fresher entries and more broadcast. Aggregation switches terminating hundreds of VLANs hit the ARP table limit long before the MAC table, and once entries are punted to software the CPU becomes the bottleneck for traffic that should be hardware-forwarded.
Formulas
ARP Table
Entries = hosts per VLAN × VLANs; refreshes per second = entries ÷ aging timeout, each one a 28-byte ARP request padded to a 64-byte frame.
- entries
- One ARP entry per reachable host per terminated VLAN
- aging timeout
- Seconds before an idle entry is revalidated — 240 s on Cisco IOS, 60 s in the Linux base_reachable_time
- 84 bytes
- 64-byte minimum frame plus 20 bytes of preamble and interframe gap
Broadcast bandwidth
ARP broadcast load = (entries ÷ timeout) × 84 × 8 bits per second.
Worked example
With the default values below, ARP Table Calculator returns ARP Entries Required of 8,000. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Hosts per VLANaccepts 1–4,000 hosts
- 200 hosts
- VLANs terminated on the deviceaccepts 1–4,094 VLANs
- 40 VLANs
- ARP aging timeoutaccepts 30–14,400 s
- 240 s
- ARP table limitminimum 1 entries
- 8,192 entries
Results
- ARP Entries Required
- 8,000
- ARP Table Used
- 97.66%
- ARP Refreshes per Second
- 33.33 req/s
- Broadcast Load from ARP
- 22.40 kbps
- ARP Broadcasts Seen per Host
- 0.83 pps
- Capacity Risk
- High — under 10% headroom in the ARP table
Step by step
- 1Values used
- 2ARP Table
- 3Broadcast bandwidth
- 4ARP Entries Required= 8,000
- 5ARP Table Used= 97.66
- 6ARP Refreshes per Second= 33.33 req/s
- 7Broadcast Load from ARP= 22.40 kbps
- 8ARP Broadcasts Seen per Host= 0.83 pps
- 9Capacity Risk= High — under 10% headroom in the ARP table
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is ARP Table calculated?
Entries = hosts per VLAN × VLANs; refreshes per second = entries ÷ aging timeout, each one a 28-byte ARP request padded to a 64-byte frame. Every host in a VLAN must process every ARP broadcast in that VLAN, so the load each host sees depends on the size of its own broadcast domain rather than the whole device. The refresh rate is set by the aging timeout — a shorter timeout means fresher entries and more broadcast.
Why does ARP Table matter?
Aggregation switches terminating hundreds of VLANs hit the ARP table limit long before the MAC table, and once entries are punted to software the CPU becomes the bottleneck for traffic that should be hardware-forwarded.
What values do I need to enter?
This calculator takes 4 inputs: Hosts per VLAN, VLANs terminated on the device, ARP aging timeout, ARP table limit. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.