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Date:	Fri, 4 May 2007 00:23:34 +0200
From:	Willy Tarreau <>
To:	Øyvind Vågen Jægtnes <>
Cc:	Jan Engelhardt <>,
	Linux Kernel Mailing List <>
Subject: Re: Routing 600+ vlan's via linux problems (looks like arp problems)

On Thu, May 03, 2007 at 11:12:09PM +0200, Øyvind Vågen Jægtnes wrote:
> On 5/3/07, Jan Engelhardt <> wrote:
> >
> >On May 3 2007 22:53, Willy Tarreau wrote:
> >>> For the rest all we see in the arp cache is (incomplete)
> >>
> >>I suspect that your arp cache is full (128 entries by default).
> >>Check /proc/sys/net/ipv4/neigh/gc_thresh1 (128 for me). You can
> >>set it as high as gc_thresh2 (512 for me), and I don't know what
> >>happens above.
> >
> >Above, you will perhaps need the not-so-elegant userspace arpd :-/
> Yes, i was suspecting that the arp cache got full, but i will try
> increasing it :)
> Would there be any huge bugs if i change these lines in arp.c:
>        .gc_thresh1 =   128,
>        .gc_thresh2 =   512,
> to
>        .gc_thresh1 =   700,
>        .gc_thresh2 =   700,
> under the definition for struct arp_tbl?

I don't think it could cause a problem, but network people will surely
correct me if I'm wrong.

> This setup will only run for about 1-2 hours while we fix the hardware
> router (it is running now, but only on a backup flash card solution.
> the harddrive in it died ;)

Huhhh! Please tell us exactly what make and model of ROUTER you are using
which embeds a HARD DRIVE, so that we recall never to buy that ! Having
seen uptimes of 5 years on moderately big access routers, I would have
find it awful to see them die multiple times in that timeframe because
of a crappy IDE drive inside !

> I have been looking at arpd, but i quickly discarded it as an option
> since its marked both experimental and obsolete ;)

I never dared to try it either, and since 512 has always been enough
for me, anything above is unknown area to me :-)


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