tcpdump was treating argument for -i option as a number if it had numeric prefix.
Packet capturing wasn't started at all on specified interface or it could get started on wrong interface.
Treat the argument to -i options as a number if it's *all*
number. Thus interface names such as 192_1_2 aren't treated as interface number 192.
tcpdump should start correctly on specified interface even if interface name has numeric prefix.