The Multicast Router Discovery (MRDISC) protocol can automatically discover multicast-capable routers. By listening to multicast router discovery messages, Layer 2 devices can determine where to send multicast source data and IGMP host membership reports. This feature is useful in a Layer 2 bridging domain that you configure for IGMP snoop.
IGMP multicast router discovery consists of three message types that discover multicast routers on the network:
Multicast router advertisements: routers advertise that IP multicast forwarding is enabled on an interface.
Multicast router solicitations: routers solicit a response of multicast router advertisements from all multicast routers on a subnet.
Multicast router termination messages: a router terminates its multicast routing functions.
Multicast routers send multicast router advertisements periodically on all interfaces where you enable multicast forwarding. Multicast routers also send advertisements in response to multicast router solicitations.
Multicast router solicitations transmit to the IGMP-MRDISC all-routers multicast group that uses a multicast address of 224.0.0.2. Multicast router solicitations do not transmit if a router needs to discover multicast routers on a directly attached subnet.
Multicast router termination messages transmit after a router terminates its multicast routing functions. Other non-IP forwarding devices, such as Layer 2 switches, can send multicast router solicitations to solicit multicast router advertisements.
To function MRDISC on IGMP snoop interface, you must explicitly enable MRDISC. The Solicitation messages are sent only if IGMP snoop and MRDISC are enabled on the switch.