I had a problem at work the other day. A customer, I do work for an ISP, had a managed router configured with several secondary subnets under their LAN gateway interface. They also had several ip helper-address commands configured to facilitate the forwarding broadcasted DHCP Discover packets to remote DHCP servers. Now the customer manages their own DHCP server, but did not have a superscope defined to accomodate the secondaries. To get around this, I enabled a feature called DHCP smart-relay.
ip dhcp smart-relay
This is a global configuration that will have the router listen for DHCPOFFER messages coming back from the helper-address that it forwarded the DHCPDISCOVER to. If it doesn't hear an offer message, it will send another discover message changing the giaddr of the DHCP packet to be that of another secondary. It will continue this behavior until it runs out of secondaries or it receives an offer message.
Now I know the above isn't a tough one to deal with for most network admins, but I was surprised that I had forgotten it and that my colleagues didn't know about it.
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