Q. Recently we made changes to our server hardware and swapped all IP address from old to a new server. However, 2 IP addresses are not get updated due to arp cache issues (IPs are cached on the router). They are going to other servers. How do I solve this problem under Redhat Enterprise Linux version 5.0 or any other Linux distribution?

A. ARP is used to resolve IP address to the corresponding Ethernet address. ARP maintains the mapping between IP address and MAC address in a table in memory called ARP cache. The entries in this table are dynamically added and removed.

This is common and well known problem. Most ISPs and network admin configure their routers with a long ARP cache timeout. As a result your requests are going to other server. If you move IP address it may take hours before server can communicate with the Internet (correct server). To get rid of this problem you need to request the MAC address for its own IP which will cause routers and other hardware update ARP cache. This is called as 'unsolicited ARP' or 'gratuitous ARP'.

There are two versions of arping, you need a version written by Alexey Kuznetsov. Both Debian and RHEL has Alexey arping command version. Type the following command to update arp cache:
# arping -U -I {Interface-Name} {IP-Address}
# arping -U -I eth1 72.2.1.2
Where,

There is also another tool called send_arp from the High-Availability Linux Project for same purpose.