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Delayed Connection to publlc devices on LAN with backup router

I have two Cisco 1921 routers, one used as a backup.  Configurations are identical.  Outfacing interface is a FastEthernet one to our ISP (Windstream).  The other interface connects to our LAN.  We have some servers with both public and private addresses on the LAN, but most devices on the LAN are PCs with private addresses.  NAT is used, but access lists are set to only NAT the private addresses. Configuration has worked fine.  However when testing the backup router by using in place of the production router, cannot get to the LAN devices with public addresses from the outside for several hours. Seems like an ARP issue, but even rebooting the server on the LAN doesn’t help.  Inserted a switch with a mirrored port in front of the router to make sure packets were getting to the router (Wireshark), and the ICMP packets are seen coming in, but no reply.  When turning off CEF and fast switching, then debugging the router, these ICMP packets (request) are not seen in the debug log until the connection comes back.  Why don’t I see the requests in the debug log? If I put the production router back into the network, the servers are again visible UNLESS I had waited long enough for a server to be visible with the backup router.  In that case, in takes several hours for the server to be visible from the outside using the production server, but the others that were never visible from the outside using the backup router are immediately visible when the production router is restored.  Help! Is there some type of dynamic config saved in these routers even when power cycled, which I always do when swapping the routers? Been pulling my hair out over this one for several weeks.

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