RE:tcpdump
You don't have to look for errors. tcpdump configures the NIC to be in promiscuous mode. See
http://en.wikipedia.org/wiki/Promiscuous_mode
I once tested a FreeBSD snapshot, and had connectivity problems with the 're' NIC . To diagnose the issue I ran tcpdump on that 're' NIC''. Mysteriously with tcpdump running, the connectivity problems (no working arp) went away..