1.) Looking at the network mask used in the interface it seems that the standby IP address is configured wrong. Also, there is not supposed to be any configurations related to the standby IP address. I mean no NAT configurations or access-list statements as its just there for the Failover to work. (To my understanding)
2.) What you should do is configure another IP from the same network range as the primary IP. When you change the standby IP address you naturally only configure it on the active device as the standby device automatically receives the configuration change from the primary device (provided that the actual failover is working properly)
3.) The IP ending in .120 cannot be used as its not from the same network range as .100 (when you are using a mask of /29) When you have a correct IP address from same network configured on the standby ASA you should be able to ping it and also see it on the "show arp" command on the ASA
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...