• mrouter (router port)
• subscriber (port belongs to established multicast group)
• none

• multicast traffic received on “none” port
  • if no mrouter present, flood to all ports
  • otherwise flood to mrouter ports
• igmp received on “none” port
  • if no mrouter present, ignore frame (no group is created)
  • otherwise
    • creates multicast group
    • forwards to mrouter ports only
    • port becomes “subscriber”
• multicast traffic received on “mrouter” port
  • send to established group members only
• igmp received on “mrouter” port
  • creates multicast group
  • forwards to mrouter ports only

• if igmp snooping is disabled, multicast traffic is flooded to all ports in vlan, regardless of frame nature (i.e. it is igmp or not); static multicast fdb entries are ignored;
• mrouters can be statically configured while igmp snooping is disabled on vlan, but they are ignored until igmp snooping is enabled; when igmp snooping is enabled, the rules listed above apply;

Analyzing the forwarding cases, it turns out that when incoming packets are forwarded to more than one port at the same time, they are forwarded to either all ports in a specific igmp group or all mrouter ports (but not a mix of the two). In other words, depending on the igmp snooping logic, either an igmp group or a list of mrouter ports is sufficient to determine which ports to forward to.

exec#

sh ip igmp snooping [vlan x]
sh ip igmp snooping groups [vlan x] [dynamic|user] [count] etc
sh ip igmp snooping mrouter [vlan x]

config#

[no] ip igmp snooping vlan x mrouter interface y
[no] ip igmp snooping vlan x static a.b.c.d interface y

• RFC 4541 - Considerations for IGMP and MLD Snooping Switches
• Configuring IGMP Snooping and MVR (Catalyst 3560)
• Mapping IP Multicast to MAC-Layer Multicast
• Some experiments with IGMP snooping behavior of a couple of switches
• Changing IGMP version on Debian Linux Systems