In Juniper routers, the Address Resolution Protocol (ARP) expiration timer does not refresh even if there is an active traffic flow in the router. This is the default behavior of all routers running Junos OS. This article includes a procedure to reset the timer to a higher value, to give all connected devices on the network time to respond.

On Juniper routers, the ARP timer update is not based on traffic flowing through the interface. When the ARP timer reaches 20 (+/- 25%) minutes, the router will initiate an ARP request for that entry to check that the host is still alive. If that request does not get a reply, the entry is flushed out of the table. This is the default behavior of all routers running Junos.

Make sure that all the connected devices are able to send a reply in the given time span. The default ARP aging timer value is 20 minutes and can be verified with the shell command. On a very big network, it is better to set the ARP timer to a higher value.

root@router% sysctl net.link.ether.inet.max_age
net.link.ether.inet.max_age: 12000

This can be changed by configuring the following knob on the CLI:

lab@router# set system arp aging-timer ?

Possible completions:

<aging-timer> Change the ARP aging time value (1..240 minutes)

lab@router# show system arp
aging-timer 10;

root@router% sysctl net.link.ether.inet.max_age
net.link.ether.inet.max_age: 6000

2020-03-04: Article reviewed for accuracy; minor changes made to keep article up-to-date