Comment 2 for bug 973609

Also, if you are using one network node, it may be getting overloaded with lease and release requests. You should probably extend the lease time to something more reasonable than 2 minutes via:

dhcp_lease_time=86400 # set the lease time to 24 hours
fixed_ip_disassociate_timeout=172800 # timeout expired leases after 48 hours

Note if you are spawning a lot of instances you will definitely want:

force_dhcp_release=true

or you will have to wait 48 hours to get your ips back and you risk using up all of them.