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
unnecessary.
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
value.
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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