Dhcp request is a broadcast and a dhcp client will accept the first offer it gets. Because the rouge dhcp server was on the same subnet as the client and it responded before the correct server, its addresses were accepted over the addresses from the correct server. When a client renews an address, it will sent a unicast request to the server since it now knows the address of the server and this is why your clients kept using the same rouge addresses.
To protect your network from this kind of thing, consider looking at features like dhcp snooping.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...