Why TTL value 1 in IGMP

Quoting 'cpt_fink':

You need something acting as the multicast querier, but not the multicast router. I do not know if this is possible on a TP-Link device. – cpt_fink Feb 18 '15 at 5:19

A workaround is possible: Add whatever cheap network device (cheap router, old network card in linux PC, etc) and send IGMP v2 or v3 queries to switch. Switch will detect that a querier is available, and send all multicast traffic to the cheap device. The eth hw of device will filter and reject the multicasts packets. In my network I captured IGMP v3 queries, and send the packets using tcpdump.

When a IGMP querier is not found, probably the switch broadcasts the multicast packets to all ports. Success Zajo Bajo

IGMP Snooping is a feature for switches to learn what multicast groups are needed on which ports. Routers not handling multicast routing don't care.

That said, without an mrouter in the network, you need to configure one (or more) igmp queriers. This ensures group membership reports are flood through the network periodically to keep the forwarding information up-to-date; otherwise it expires and traffic is either dropped, or floods everywhere.

The benefit is that multicast traffic goes only where it belongs. If nothing on that port has joined that group, the switch will prune it. In a cascaded environment, that can significantly reduce inter-switch traffic. And likewise keep access links clear of traffic hosts don't want.

If you don't know it's enabled, situations may arise wherein traffic may appear to be "missing" on the wire, because it's being pruned. (fast-leave, no querier, no joins, etc.)

Thus, it's rarely enabled by default.