When I enable staticroute On server (NetBSD-3) 
to bind MAC address with IP address, I found a problem:

Once clients don't communicate to server for some time (e.g. 30min),
the server will ping clients failed.
With tcpdump, I can see: server send icmp packet without arp request. 
clients are NetBSD or Linux. I notice that there are no arp entry of server.
So they ignore the icmp request???

1) server should broadcast its arp periodcally? how??
OR
2) clients generate arp entry auto once they receive an IP packet? how??
OR
3) clients configure static arp entry of server. I know how to do it, but
I don't like this method.

I perfer the method 1, because it can anti ARP-spoof on a LAN.
method 3 is the best for ARP-spoof, but there are much configuration
works to do.