What Is the Cisco Default ARP Timer and Should You Change It?
Selene Gong
The Address Resolution Protocol (ARP) is critical for resolving IP addresses to MAC addresses within Layer 2 and Layer 3 networks. In Cisco devices, ARP entries are stored in a cache with a predefined aging timer. The Cisco default ARP timer is typically set to 14400 seconds (4 hours). Understanding this timer is essential for network engineers managing dynamic switching environments, as mismatched ARP and MAC aging values can lead to performance issues such as unicast flooding or stale mappings.
This article explains what the Cisco default ARP timer is, how it works, and when it should be adjusted to optimize network performance.
What Is the Cisco Default ARP Timer?
Cisco routers and switches use an ARP cache to store recently resolved IP-to-MAC address mappings. Each entry in the ARP cache is subject to a timeout known as the ARP timer, which defines how long a mapping remains valid before it is removed or refreshed.
Default ARP timeout: 14400 seconds (4 hours)
Purpose: Prevent outdated mappings from remaining in the ARP table
Applies to: All Cisco IOS-based devices including Catalyst switches and ISR routers
When the ARP timer expires, the device sends a new ARP request to confirm or update the mapping.
Why ARP Timer Settings Matter
The ARP timeout plays a critical role in maintaining accurate network mappings and minimizing unnecessary traffic. Here's why the setting matters:
1. MAC Aging vs. ARP Aging
If the MAC address aging timer is shorter than the ARP timer, a switch may remove a MAC entry from its forwarding table while still retaining the corresponding ARP entry. This causes packets to be flooded until the MAC address is relearned.
For example:
ARP timeout = 14400s
MAC aging = 300s
This mismatch leads to Layer 2 flooding after 5 minutes.
2. VoIP, HA, and Mobile Environments
In networks with frequent topology changes or high-availability configurations, long ARP timers can delay reconvergence. VoIP traffic in particular is sensitive to stale address entries.
3. Control Plane Load
Frequent ARP refreshes (if timer is too low) can generate excessive broadcast traffic, increasing CPU usage on switches and routers.
How to View and Configure ARP Timer in Cisco IOS
View Current ARP Timeout
To check ARP timer values on a Cisco interface:
show interfaces | include ARP
Example output:
GigabitEthernet0/1 is up, line protocol is up
ARP type: ARPA, ARP Timeout 04:00:00
Configure ARP Timeout on an Interface
To set the ARP timeout manually:
configure terminal
interface GigabitEthernet0/1
arp timeout 600
end
Value is in seconds
Minimum: 60 seconds
Maximum: 2147483647 seconds
Use write memory to save the configuration if needed.
Use Cases: Cisco Catalyst Switches and ISR Routers
When to Modify the ARP Timer
Use Case 1: Access Layer with Dynamic Clients Lowering the ARP timeout (e.g., to 300–600s) helps adapt to changing MAC addresses in environments with laptops or mobile devices.
Use Case 2: VoIP and Real-Time Applications Shorter ARP timers ensure faster address updates and reduce the impact of endpoint movement or failure.
Use Case 3: L3 Core with Static Devices Longer ARP timers reduce overhead and are acceptable when endpoints are static (e.g., servers or printers).
Best Practices for ARP Timer Configuration
Align ARP timeout with MAC aging
Avoid ARP timers significantly longer than MAC aging
Common pairing: ARP timeout 600s / MAC aging 300s
Monitor ARP behavior
Use SNMP, show ip arp, or logging to detect excessive ARP requests
Test before deploying changes
Adjust timers gradually and observe traffic behavior
Document timer values
Maintain consistency across interface configurations
Frequently Asked Questions (FAQ)
Q1.What is the default ARP aging time on Cisco switches?
The default ARP aging time on most Cisco devices is 14400 seconds (4 hours).
Q2.How to check ARP timer settings in Cisco IOS?
Use the command:
show interfaces | include ARP
This displays the ARP timeout per interface.
Q3.Should I modify the ARP timeout on a Cisco Catalyst switch?
Yes, in certain situations such as mismatched MAC aging, VoIP deployment, or dynamic endpoint environments, reducing the ARP timeout improves performance.
Q4.What happens if ARP timeout is too short or too long?
Too short: Increases broadcast ARP traffic, potentially raising CPU usage
Too long: Causes unicast flooding if MAC table ages out before ARP entry
Q5.What does "ARP age time 0" mean?
Setting ARP age time to 0 disables aging, meaning entries never time out. This is not recommended in dynamic networks.
Conclusion
The Cisco default ARP timer of 14400 seconds works well in many environments, but it is not always optimal. Adjusting this setting to match MAC aging or specific network conditions can prevent performance issues and reduce flooding. Always align your ARP and MAC timers, especially on Layer 3 switches and in dynamic environments such as VoIP or high-availability networks.
