9.5 Configure IP Multicast Routing
9.5.5
Select a designated router

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.