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

[Nitzan Tzelniker]

>From what I understand the router will not delete the arp entry immediately
after it  expired  so it will not queue/drop the packet
Take a look on this output where the arp is expired you the entry is kept
without expiration time for few seconds until the other side answer to the
arp

nitzan at ROUTER> show arp no-resolve expiration-time | match 10.10.3.58
00:02:ff:10:44:7c 10.10.3.58     ae0.21                 none   1

nitzan at ROUTER> show arp no-resolve expiration-time | match 10.10.3.58
00:02:ff:10:44:7c 10.10.3.58      ae0.21                    none

nitzan at ROUTER> show arp no-resolve expiration-time | match 10.10.3.58
00:02:ff:10:44:7c 10.10.3.58      ae0.21                    none  90

Nitzan

On Fri, Jan 11, 2019 at 6:51 PM Clarke Morledge <chmorl at wm.edu> wrote:

> 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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