ipv4: Neighbour table overflow

I was noticing the below error message in /var/log/messages. I also noticed that my linux DHCP server was not serving leases. Again, I modified the sysctl file to fix the issue.

Problem:

Jun 21 07:56:03 localhost kernel: [36050.926734] ipv4: Neighbour table overflow.

Jun 21 07:56:03 localhost kernel: [36050.930354] ipv4: Neighbour table overflow.

Jun 21 08:01:02 localhost kernel: [36349.592127] ipv4: Neighbour table overflow.

Solution:

1. Edit /etc/sysctl.conf

 vi /etc/sysctl.conf2. Change to have the following values

net.ipv4.neigh.default.gc_thresh1 = 8192

net.ipv4.neigh.default.gc_thresh2 = 16384

net.ipv4.neigh.default.gc_thresh3 = 16384

2. To load new changes type the following command:

  sysctl -p

What are these 3 parameters?

These parameters are defined in the Linux Kernel Code in /include/net/neighbour.h as integer, which suggests that maximal accepted value is (2^32 - 1)

gc_thresh1 - The minimum number of entries to keep in the ARP cache.

gc_thresh2 - The soft maximum number of entries to keep in the ARP cache. 

gc_thresh3 - The hard maximum number of entries to keep in the ARP cache.