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IGMP Snooping / Forwarding Behaviour

Port states

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;

exec#

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