How to capture src and dst traffic (both directions) with tcpdump, with one command

Host filtering in tcpdump works on MPLS-prefixed packets if you specify "mpls" before the "host" primitive.

$ tcpdump 'mpls and host example.com'

or

$ tcpdump 'host example.com or (mpls and host example.com)'

if you're dealing with packets with and without the MPLS prefix.

As you pointed out, there is nothing in the documentation about the "packets dropped by interface" counter. So we need some source code digging.

From the source code of tcpdump, the interface drop counter is extracted from stats.ps_ifdrop:

if (stats.ps_ifdrop != 0) {
    if (!verbose)
        fputs(", ", stderr);
    else
        putc('\n', stderr);
    (void)fprintf(stderr, "%u packet%s dropped by interface\n",
        stats.ps_ifdrop, PLURAL_SUFFIX(stats.ps_ifdrop));

From man pcap_stats:

ps_ifdrop
    number of packets dropped by the network interface or its driver.

And from the libpcap source code:

 *  "ps_ifdrop" is supported. It will return the number
 *  of drops the interface reports in /proc/net/dev,
 *  if that is available.

So the tcpdump "packets dropped by interface" counter corresponds to the packets logged as dropped in /proc/net/dev during the tcpdump capture.

The meaning of the /proc/dev/net fields are explained here

To get a better understanding of the drops, I would start by looking at the following statistics:

• ethtool -S <interface>
• grep '' /sys/class/net/<interface>/statistics/*