These requests are innocuous and completely expected. Plex uses the Simple Service Discovery Protocol (SSDP) to discover other Plex Media Servers, networked TV tuners, players, etc… on the local network. To do so, it sends out search requests. For those devices which respond, additional queries are sent to find out if they’re devices in which Plex is interested. Here’s a packet capture showing the search/response flow:
As far as I can tell, this repeats every ten seconds or so, and there’s no way to completely disable it as some of Plex’s essential functions require it. There’s a setting under Settings -> Network called Enable local network discover (GDM) which may impact it to an extent. You could block the SSDP port (UDP 1900) on your network (or in your PC firewall), but I’m fairly certain that would break some things that normally happen automatically. Or, perhaps, you could just block UDP port 1900 for those devices for which Plex will never need access. UPnP also uses SSDP for its underpinnings; so, disabling any UPnP functions of devices on your network may serve to reduce the amount of SSDP responses received from PMS’ queries.
To sum up though, Plex Media Server isn’t indiscriminately pulling data from devices on your network. It’s part of a search/response/query flow intended to find media-related devices. So, there is a good reason this traffic exists and it’s essential to some of Plex’s functions.
