You will have to NAT the traffic before it hits the concentrator. This gets a little tricky. You wont be able to NAT this on the concentrator so you'll have to do that on a router or a pix. So you got to question that as well since the pix and router could do both the nat and the ipsec tunnel. Is the concentrator needed is what i'd be asking. To answer your questions directly:
1. Not on the concentrator, so it will have to be done before it goes through the tunnel on another device.
2. NAT happens before encryption, whether its on the vpn device or downstream. No it wont arp for the old address.
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...
[toc:faq]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 and UDP are I...