Security Incidents: against illegal arp update

["SB CH" <chulmin2 () hotmail com>]

Hello, all.

Today someone(I don't know who) which use same network(/20), has updated 
our arp information into non-existent mac information. so I can't connect 
toward the server.
the number of the servers which can't connect are so high, so I can't set 
all ips into mac at the router or switch.
Surely, I use arpwatch. But I can't find any information about it.
I know that one can fake his ip and update illegal arp information against 
some ip which is a same network.
Is it a virus or illegal attack? 
How can I solve this incident?
Thanks in advance.





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