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.

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.

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