[c-nsp] ARP strangeness

[Keegan Holley]

>
> We have one of those NMS systems that periodically "reads L2 devices for
> mac-address/port mapping" and "reads L3 devices ARP for mac-to-IP
> mapping".  Ideally, there should be no missing links (if the MAC is
> found, hopefully the ARP/IP is found, and vice-versa).
>
> For the default mac-address aging time of 300 seconds, I had this notion
> that setting the ARP timeouts to 270 seconds would necessitate the
> router ARPing the device (assuming active traffic) prior to the
> mac-address aging out, keeping the mac-address table populated.
>
> But if the Cisco L3 behavior is to gratuitously do this for me before
> the ARP timeout... that changes things a bit.
>
> Is this default behavior across all the Cats, or just the 6500/7600?  Is
> it supervisor-specific?
>

If I'm not mistaken this or something similar is standard behavior for
several different vendors now.  I remember asking this question after
opening a TAC case.  They said the problem is there is a chance of high
latency or even packet loss during the arp process and this would happen
every arp interval.  Also, because of the way CEF works a large number if
not all of the arp entries would time out at the same time each interval
which could lead to cpu spikes.