Linux – Sniff packets using tcpdump

It looks like it is your Cisco switches running the Spanning Tree Protocol. (I did a look up 00:15:c6 is a Cisco MAC and 01:00:0c:cc:cc:cd is the multicast MAC address Cisco Shared Spanning Tree Protocol).

If you don't have any loops in your switch topology you could turn it off, but I doubt it will impact performance.

tcpdump prints complete packets. "Garbage" you see are actually TCP package headers.

you can certainly massage the output with i.e. a perl script, but why not use tshark, the textual version of wireshark instead?

tshark 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

it takes the same arguments as tcpdump (same library) but since its an analyzer it can do deep packet inspection so you can refine your filters even more, i.e.

tshark 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)' -R'http.request.method == "GET" || http.request.method == "HEAD"'