Cisco routers use the PIM routing protocol to
forward multicast traffic. Two PIM routers are neighbors if there is
a direct connection between them.
Each multicast-enabled router is configured to
know which interfaces will be using PIM. These interfaces connect
either to neighboring PIM routers in the network or to host systems
on a LAN. If there are multiple PIM routers on a LAN, they are
considered to be neighbors.
The procedure for electing the designated
router (DR) is the same for both PIM SM and PIM DM. Each PIM router
connected to a multiaccess LAN issues periodic PIM router queries
onto the LAN. The PIM router with the highest IP address becomes the
DR for the LAN. If the DR becomes inoperable, a new DR is elected
from the alternate PIM routers on the LAN.
Note: DR election is necessary only on
multiaccess networks. It is not required for point-to-point links
because the connected router is effectively the DR for directly
connected host systems.
On a multiaccess LAN, one router is selected
to poll the LAN for host group membership. However, all PIM routers
on a single subnet receive replies from the host. The router
selected to poll the LAN is called the DR. The DR is responsible for
sending IGMP host-query messages to all hosts on the LAN. There can
be many DRs in a campus network, one for each leaf subnet of the
distribution tree.