[c-nsp] ARP strangeness

[Frank Bulk - iName.com]

The 7609 does stop ARPing after receiving a reply from the CPE, but the 7609
ARPs again 7 minutes later.  One person told me off-list that Cisco doesn't
expire an ARP entry before checking its ARP entries by doing an ARP request.
Since ARP timeout is set for 8 minutes, perhaps Cisco's approach is to ARP
the host one minute before expiration.

Because the FTTH product has its own smart-L2 implementation with a MAC
address expiration time of 7 minutes, it won't forward directed or broadcast
ARP requests from the 7609 once the CPE's MAC address has expired from the
FTTH's MAC table.  Once the CPE goes deaf, even a full DHCP exchange doesn't
"wake up" the connection.  Only power-cycling the BEFRS41 resolves the
issue.  The difference between a power cycle and a full DHCP exchange is
that the BEFSR41 does an ARP request for the default router (7609) after the
DHCP exchange.

I've extended the MAC address expiration time of the FTTH link to 15 minutes
(two times the 7 minutes plus 1 minute) and we'll see how that goes.

Frank

-----Original Message-----
From: Keegan Holley [mailto:keegan.holley at sungard.com]
Sent: Monday, January 03, 2011 7:14 PM
To: <frnkblk at iname.com>
Cc: <cisco-nsp at puck.nether.net>
Subject: Re: [c-nsp] ARP strangeness

The 7600 should stop arping if it has gotten a reply.  What happens when you
ping your test CPE both from the router and from another node behind it?
You can also run he command sh ip cef exact-match to see what the router is
doing with a specific source-destination ip pair. If nothing strange pops up
then it's probably a bug.

Sent from my iPhone

On Jan 3, 2011, at 12:58 PM, Frank Bulk <frnkblk at iname.com> wrote:

> 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
>
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