On ARP and MAC Aging Timers

Key Takeaways

• Arista defaults: ARP timeout 4 hours, MAC aging 5 minutes.
• ARP timeout reflects early low‑memory, low‑CPU network constraints.
• MAC aging clears stale bridge entries, preventing misdirected traffic.
• Longer ARP entries rely on Gratuitous ARP for address changes.
• EVPN adds control plane, reducing need for aggressive MAC aging.

Pulse Analysis

The four‑hour ARP timeout and five‑minute MAC aging timer originated in the 1980s, when Ethernet ran over thick coax and devices had only a few megabytes of RAM. Designers prioritized minimizing broadcast storms; a long ARP cache reduced the need for frequent requests, while a relatively short MAC aging period prevented stale bridge entries from causing frames to be sent down the wrong port. These values became de‑facto standards and were baked into many vendor defaults, including Arista’s EOS.

In today’s data‑center, the same defaults persist, but the network landscape has dramatically changed. Virtual switches, 10‑GbE and higher links, and EVPN overlay fabrics introduce a control plane that can distribute MAC address information without relying on dynamic learning alone. Nevertheless, the ARP timeout still governs how long hosts trust cached IP‑to‑MAC mappings, and gratuitous ARP remains the primary mechanism for updating those entries. An overly aggressive ARP timeout can increase broadcast traffic, while a lax setting may delay detection of address hijacks.

Network operators should evaluate these timers against their specific workloads. For environments with frequent VM migrations or mobile endpoints, shortening the ARP timeout can improve address accuracy, but must be balanced against the added broadcast load. Conversely, extending MAC aging beyond the default may reduce unnecessary flooding on high‑speed links, yet risks retaining obsolete entries that could misroute traffic. Adjusting the timers with awareness of the historical trade‑offs ensures optimal performance and security in modern Ethernet deployments.