The
switch periodically sends IGMP host-query
messages to discover which multicast groups are present on attached networks.
These messages are sent to the all-hosts multicast group (224.0.0.1) with a
time-to-live (TTL) of 1. The
switch sends host-query messages to refresh
its knowledge of memberships present on the network. If, after some number of
queries, the software discovers that no local hosts are members of a multicast
group, the software stops forwarding multicast packets to the local network
from remote origins for that group and sends a prune message upstream toward
the source.
The
switch elects a PIM designated router (DR)
for the LAN (subnet). The DR is the router or multilayer
switch with the highest IP address for
IGMPv2. For IGMPv1, the DR is elected according to the multicast routing
protocol that runs on the LAN. The designated router is responsible for sending
IGMP host-query messages to all hosts on the LAN. In sparse mode, the
designated router also sends PIM register and PIM join messages toward the RP
router.
This procedure is optional.