Re: Neighbour table overflow problem
Jeff S Wheeler wrote:
> Dear list,
>
> I have a linux 2.4 box running zebra and acting as a default
> gateway for
> a number of machines. I am concerned about "Neighbour table overflow"
> output in my dmesg. From some articles I've read on usenet, this is
> related to the arp table becoming full. Most of the posters solved
> their problems by configuring a previously unused loopback
> interface, or
> realizing that they had a /8 configured on one IP interface
> and a router
> on their subnet that was using proxy-arp to fulfill the arp requests.
>
This was true with some redhat release (unconfigured loopback). But as you
realized, this is not your problem.
> When the network is busy I've seen as many as 230+ arp
> entries, but it never seems to break 256. Is this an artificial limit
> on the number of entries that can be present in my arp table?
No, it's just the default (guess: 254 ARP entries per NIC?). Had the same
problem, one (err, actually two; different story) machine acted as default
router for various /22 subnets on each of the 4 NICs. It was just massive
ARP requests, as we got our networks from previously shut down ISP. I guess
even unfulfilled ARP request take a slot in the kernel neighbor table.
> If so, I
> would like to increase the limit by to 2048 or so and give myself some
> headroom. I am concerned that might slow down packet
> forwarding, but I can probably live with that.
After some googling, I found:
echo 2048 > /proc/sys/net/ipv4/neigh/default/gc_thresh3
Up to now I found no drawbacks, "only" the kernel message is gone.
>
> Has anyone on the list encountered similar problems? If so,
> is this the
> approach you took to solve them or did you do something else?
>
> Thanks,
>
> --
> Jeff S Wheeler <jsw@five-elements.com>
Thomas
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