What are the benefits and cons of IGMP snooping?

Question

Our new router is asking if we want IGMP snooping enabled. Not being familiar with this, I looked to the internet and got the following wiki description:

IGMP snooping is the process of listening to Internet Group Management Protocol (IGMP) network traffic. The feature allows a network switch to listen in on the IGMP conversation between hosts and routers. By listening to these conversations the switch maintains a map of which links need which IP multicast streams. Multicasts may be filtered from the links which do not need them and thus controls which ports receive specific multicast traffic.

But what are the benefits? And what considerations (security, performance, etc) need to be taken into account before enabling it? My view is - if it's safe, then why isn't just on by default?

Answer 1

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.