Perryg, thanks for your help thus far - you have been a great resource.
I am working with Bryan on this project. We are not using NAT. Instead, what we are seeing is a flurry of duplicate packets, regardless of whether the packets are ICMP echoes or standard TCP messages. My suspicion is that internal networking within VirtualBox is not getting along with Dynamips, which we are using to route between internal networks.
Here is an example of what is happening. We have a two Dynamips routers, both with an interface on an internal network named "InternalNet". One host is present on "InternalNet". Router1's interface has address 10.0.0.1, Router2's interface has address 10.0.0.254, and the host has address 10.0.0.100. When I ping 10.0.0.1 (Router1) from the host, I get a bunch of duplicate replies from - here is the crazy part - 10.0.0.254 (Router2). That's right - Router2 is responding to pings addressed to Router1!
Any ideas on what is causing this? My theory is that the virtual networking technique used by VirtualBox is conflicting with the technique used by Dynamips. From what I can tell, the Dynamips router interface uses a MAC address not bound to the interface in the guest OS, so some voodoo has to go on for the Dynamips router to "see" traffic destined to it. Whatever is going on, it is complicated, and it is starting to hurt my brain
Any help would be greatly appreciated!