Kernel Neighbour table overflow

Sometimes we may see this kind of messages in /var/log/messages


Oct 31 11:15:38 mail kernel: [ 4897.071608] __ratelimit: 97 callbacks suppressed
Oct 31 11:15:40 mail kernel: [ 4898.932524] Neighbour table overflow.

The neighbour table is generally known as ARP table and the default value for gc_thresh1 is 128.

Check the present value with the following command and the result should be like this:

# cat /proc/sys/net/ipv4/neigh/default/gc_thresh1

128

the 128 value is not enough. We also need to change the gc_thresh2 (soft limit) and gc_thresh3 (hard limit) for this. 

To resolve this problem edit the /etc/sysctl.conf file and add the following lines there.

net.ipv4.neigh.default.gc_thresh1 = 4096

net.ipv4.neigh.default.gc_thresh2 = 8192
net.ipv4.neigh.default.gc_thresh3 = 8192
net.ipv4.neigh.default.base_reachable_time = 86400
net.ipv4.neigh.default.gc_stale_time = 86400

then check with #sysctl -P

Hope the problem will be solved.