[j-nsp] Junos Arp Expiration Timer Behavior & Active Flows

Clarke Morledge chmorl at wm.edu
Fri Jan 11 11:50:52 EST 2019

According to KB19396, "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." The default timer is 20 minutes. I have confirmed this behavior on 
the MX platform.

This does not seem very intuitive, as it suggests that a Junos device at 
L3 would stop in the middle of an active flow, to send an ARP request to 
try to refresh its ARP cache, potentially causing some unnecessary queuing 
of traffic, while the Junos device waits for ARP resolution. For an active 
flow, the ARP response should come back quick, but still it seems 

I would have thought that the ARP cache would only start to decrement the 
expiration timer, when the device was not seeing any traffic to/from ARP 
entry host.

KB19396 goes onto say, "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." I can see that when the ARP timer is started 
initially, that it starts the expiration countdown, at this (+/- 25%) 
value, and not exactly at, say, 20 minutes, which is the default timer 

A couple of questions:

(a) Is this default behavior across all Junos platforms, including MX, 
SRX, and EX?

(b) Is there any other caveat as to when the Junos device will send out 
the ARP request, at the end of expiration period?

Clarke Morledge
College of William and Mary
Information Technology - Network Engineering
Jones Hall (Room 18)
200 Ukrop Way
Williamsburg VA 23187

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