Multicast-DR Election

The designated router for a LAN is the only router that sends IGMP host-query messages.
PIM hello messages are also used to elect a designated router for each multi-access network. The DR is the router that will forward the PIM join message from the receiver to the RP (rendezvous point). The DR is responsible for sending Join/Prune message to the RP.
The election process is based on the highest priority and highest IP address; the router with the numerically higher value wins the election. This process is preemptiveand every new router with a better priority will preempt the previous DR.
When the DR hears multicast packets on the segment, it will check to determine whether the destination group has an RP. If it does, the data packets are encapsulated into special PIM Register messages and sent to the RP. The RP will start forwarding them down the shared tree if there are any active subscribers. At the same time, the RP will build a shortest-path tree toward the DR and send a PIM Register-Stop message to the DR to inform it that regular forwarding may start now. After this, the multicast traffic is delivered over the SPT. 


R1(config-if)#
*Mar  1 00:01:50.123: PIM(0): Check DR after interface: FastEthernet0/0 came up
*Mar  1 00:01:50.123: PIM(0): Changing DR for FastEthernet0/0, from 0.0.0.0 to 12.1.1.1 (this system)
*Mar  1 00:01:50.123: %PIM-5-DRCHG: DR change from neighbor 0.0.0.0 to 12.1.1.1 on interface FastEthernet0/0
R1(config-if)#
*Mar  1 00:01:50.247: PIM(0): Building Graft message for 224.0.1.40, FastEthernet0/0: no entries
R1(config-if)#
*Mar  1 00:02:11.163: %PIM-5-NBRCHG: neighbor 12.1.1.2 UP on interface FastEthernet0/0
*Mar  1 00:02:11.279: PIM(0): Update FastEthernet0/0/12.1.1.2 to (*, 224.0.1.40), Forward state, by PIM *G Join
*Mar  1 00:02:11.287: PIM(0): Changing DR for FastEthernet0/0, from 12.1.1.1 to 12.1.1.2
*Mar  1 00:02:11.287: %PIM-5-DRCHG: DR change from neighbor 12.1.1.1 to 12.1.1.2 on interface FastEthernet0/0
R1(config-if)#
*Mar  1 00:02:11.363: PIM(0): Building Graft message for 224.0.1.40, FastEthernet0/0: no entries

R2(config-if)#
*Mar  1 00:02:09.883: %PIM-5-NBRCHG: neighbor 12.1.1.1 UP on interface FastEthernet0/0
R2(config-if)#
*Mar  1 00:02:09.999: PIM(0): Update FastEthernet0/0/12.1.1.1 to (*, 224.0.1.40), Forward state, by PIM *G Join
*Mar  1 00:02:10.007: PIM(0): Building Graft message for 224.0.1.40, FastEthernet0/0: no entries
R2(config-if)#
*Mar  1 00:02:11.839: PIM(0): Check DR after interface: FastEthernet0/0 came up!
*Mar  1 00:02:11.839: PIM(0): Changing DR for FastEthernet0/0, from 0.0.0.0 to 12.1.1.2 (this system)
*Mar  1 00:02:11.839: %PIM-5-DRCHG: DR change from neighbor 0.0.0.0 to 12.1.1.2 on interface FastEthernet0/0
R2(config-if)#
*Mar  1 00:02:11.967: PIM(0): Building Graft message for 224.0.1.40, FastEthernet0/0: no entries



Changing the dr priority on R1

R1(config)#int f0/0
R1(config-if)#ip pim dr-priority 100
*Mar  1 00:12:44.243: PIM(0): Changing DR for FastEthernet0/0, from 12.1.1.2 to 12.1.1.1 (this system)
*Mar  1 00:12:44.247: %PIM-5-DRCHG: DR change from neighbor 12.1.1.2 to 12.1.1.1 on interface FastEthernet0/0