Find out the addresses of multicast mailings on the router?

Question

Computer networks

There is a tp-link adsl 2+ router, with one port, I connect a set-top box to this port, a TV multicast mailing is loaded and shows the broadcast, how can I find out this address, open-wrt there is no firmware for my router. tcpdump on the router with writing to a file

Answer 1

The simplest option is to connect a stupid switch to the port and a set-top box and a computer together into it, multicast traffic will go to both the computer and the set-top box, watch with wireshark / tcpdump