ARP Table Calculator
Size an ARP cache across VLANs and estimate the broadcast load its refresh cycle puts on every host.
Inputs
ARP Entries Required
8,000
ARP Table Used
97.66%
ARP Refreshes per Second
33.33req/s
Broadcast Load from ARP
22.40kbps
ARP Broadcasts Seen per Host
0.83pps
Capacity Risk
High — under 10% headroom in the ARP table
Step by step
Values used
Hosts per VLAN = 200 hosts; VLANs terminated on the device = 40 VLANs; ARP aging timeout = 240 s; ARP table limit = 8,192 entries
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.
Broadcast bandwidth
ARP broadcast load = (entries ÷ timeout) × 84 × 8 bits per second.
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
How it works
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.
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.