IP address is not routed correctly

Run tcpdump on the main node to ensure that packets are at least reaching the physical server. Run it with the -e flag to see the MAC address as well.

If the MAC address is wrong (i.e. doesn't match what's in the Xen configuration), run arping -c 1 -S IP address Gateway address from inside the VPS to invalidate and correct the router's ARP cache entry.