PIM DR

On a multiaccess network there may be multiple IGMP enabled routers. It is the responsibility of one of these IGMP routers to send any PIM join messages towards the RP.

If no PIM DR priority is expilicitly configured the IGMP/PIM router with the highest ip address is elected as the DR and will send the join. The PIM DR priority can be used to influence which router is elected to forward the PIM join messages.

In the above scenario, without any DR priorities configured, R6 is elected DR as it has the highest ip address.

Router_1>S IP PIM NE
PIM Neighbor Table
Mode: B - Bidir Capable, DR - Designated Router, N - Default DR Priority,
S - State Refresh Capable
Neighbor Interface Uptime/Expires Ver DR
Address Prio/Mode
155.1.148.6 FastEthernet0/0 00:01:26/00:01:17 v2 1 / DR S
155.1.148.4 FastEthernet0/0 00:02:16/00:01:18 v2 1 / S
155.1.0.5 Serial2/0 00:00:21/00:01:24 v2 1 / DR S


If the lab requirement states R1 should be the DR for this segment this can be achieved with the use of the 'ip pim dr-priority' message.

config#int fa0/0
config-if#ip pim dr-priority 100

With the above config applied i re-examine the PIM neighbors and R1 has pre-empted the DR position.

Router_4#s ip pim ne
PIM Neighbor Table
Mode: B - Bidir Capable, DR - Designated Router, N - Default DR Priority,
S - State Refresh Capable
Neighbor Interface Uptime/Expires Ver DR
Address Prio/Mode
155.1.148.6 FastEthernet0/0 00:02:14/00:01:28 v2 1 / S
155.1.148.1 FastEthernet0/0 00:02:35/00:01:28 v2 100/ DR S
155.1.46.5 Serial2/1 00:02:49/00:01:22 v2 1 / S

From R1 this can be seen as well using the 'show ip pim interface fa0/0' command.

Router_1#s ip pim interface fa0/0
Address Interface Ver/ Nbr Query DR DR
Mode Count Intvl Prior
155.1.148.1 FastEthernet0/0 v2/SD 2 30 100 155.1.148.1

In summary the PIM DR controls upstream PIM joins, and from my previous post the PIM assert mechanism controls downstream routing of multicast traffic.