[c-nsp] ARP strangeness

[Frank Bulk]

We have over a thousand FTTH customers hanging off a VLAN on our 7609-S
running 12.2SRE3.  Those who have Linksys BEFRS41 (wired-only routers) are
complaining about lack of Internet access after many hours or days of idle
time (not using their PC or other devices).  Those who have Linksys WRT54G
(wireless) have no complaints (my guess is that they're sending packets out
regularly).

We replicated this in our CO and put a hub between the ONT and the Linksys
CPE so that we could capture those packets.  What we're seeing in that
capture are directed ARP requests every 7 minutes from the 7609 to the
Linksys with an ARP response from the Linksys.  After many hours, the 7609-S
stops sending the ARP requests (well, at least we're not seeing it come in,
perhaps it did try).  

We currently have our ARP timeout set to 480 seconds and MAC address table
aging time to 540 seconds.  Why?  We use "mac-address-table synchronize"
which is set to 160 seconds by default.  The recommendation from that
command is to set ARP three times that, so that would be 480.  But it's also
recommended that the MAC address table aging time be greater than the ARP
timeout, so we added another 60 seconds on top.

Two questions: 
- why is the 7609 sending any directed ARP requests at all, every 7 minutes?
- why does it appear to stop sending them after many hours?

I'm all ears if we should be using different expiration values, but the
numbers I'm using are based on reading a lot of cisco-nsp archives and Cisco
tech articles.

Frank