Are you able to post some more info such as the config on either sides (without sensitive info). It would also be useful if you could do a "show crypto isakmp sa" and "show crypto ipsec sa" on the cisco device. May be worth capturing some debugging too.
Is there any NAT going on between the two devices? Or do the external interfaces of both have public IP addresses.
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...