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Multicast routing basics & configurations

The multicast routing is applicable in IPTV, online gaming and Video conference. Now what is the logic behind the multicast routing.

There are two methods of multicast routing protocol. The PIM SM & PIM DM. these multicast protocols are applicable on different scenarios.

PIM SM (Protocol Independent Multicast Sparse Mode)

This method of multicast uses assert mechanism. The application of PIM SM is on a topology where there are few number of routers between the source and the group member of that multicast.


The dense mode uses flood and prune mechanism. The application of the PIM DM is in topologies where multicast listeners are densely populated.

Multicast routing principle

In PIM DM prune message is sent to the upstream device through unicast to join the multicast group. Then prune acknowledgement is returned to the downstream device through unicast.

PIM selects the DR router with the interface connected by the large IP address. For query messages the default query message is 60s. This can be shortened for fast multicast route learning.


IGMP is configured on the downstream to the user gateway.

How is IGMP configured

  1. Configure multicast on L3 network

  1. First establish unicast communication between the multicast source router and the downstream RP routers using unicast routing protocol. Mostly OSPF is used for this purpose.

  2. Configure PIM and RP on the upstream devices.

  3. Enable IGMP in interfaces connected to the user gateway devices.

  4. PIM protocol generate multicast route based on IGMP group information and forwards multicast data to the receiver network segments.

  1. Configure multicast on L2 networks.

  1. Configure IGMP snooping in VLANs.

Configuration Demonstration

This scenario VLAN 101 & 102 can be configured with different IGMP group. The VLAN 201, 202 & 203 can be configured with PIM SM. The RP is the router connected to the source multicast router.


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