Which two reasons explain why a server on VLAN 10 is unable to join a multicast stream that originates on VLAN 20? (Choose two.)

Which two reasons explain why a server on VLAN 10 is unable to join a multicast stream that originates on VLAN 20? (Choose two.)
A . IGMP snooping and mrouter are not enabled on VLAN 10.
B . VLAN 20 has no IGMP snooping querier defined and VLAN 10 has no mrouter.
C . The mrouter on VLAN 20 does not see the PIM join.
D . The mrouter must be on VLAN 10 and VLAN 20.

View Answer

Answer: A, C

Explanation: IGMP snooping is a mechanism to constrain multicast traffic to only the ports that have receivers attached. The mechanism adds efficiency because it enables a Layer 2 switch to selectively send out multicast packets on only the ports that need them. Without IGMP snooping, the switch floods the packets on every port. The switch "listens" for the exchange of IGMP messages by the router and the end hosts. In this way, the switch builds an IGMP snooping table that has a list of all the ports that have requested a particular multicast group.

The mrouter port is simply the port from the switch point of view that connects to a multicast router. The presence of at least one mrouter port is absolutely essential for the IGMP snooping operation to work across switches.

All Catalyst platforms have the ability to dynamically learn about the mrouter port. The switches passively listen to either the Protocol Independent Multicast (PIM) hellos or the IGMP query messages that a multicast router sends out periodically.

Reference: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/68131-cat-multicast-prob.html