Multicast PIM Sparse-Dense Mode

This topic is to discuss the following lesson:

Hi Rene.
Thank for great explain, How ever i still confuse. Can you explain what is RP mapping agent ? and why we need it ?

Hello Heng

In the first version of PIM-SM, all routers were required to be manually configured with the IP address of the RP. This does not scale well for a large complex network.

After this, Cisco implemented the Auto-RP feature which automates the distribution of group-to-RP mappings in a PIM network. To make Auto-RP work, there must be an RP mapping agent which receives the RP announcement messages from the RPs and arbitrates conflicts. The RP mapping agent then sends the consistent group to RP mappings to all other routers by dense mode flooding. This way, all routers automatically discover which RP to use for the groups they support.

I hope this has been helpful!


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I have a very basic topology of router on a stick so two VLANS (10 and 20) can communicate. What type of multicasting (SPARSE / DENSE) should I configure on my router (C2900 V15.2) to enable a server in one VLAN (VLAN 10) to send multicast traffic to devices in its own VLAN and devices in the other VLAN (VLAN 20) - that is connected via Router on a stick (sub – interfaces)

Hello Jason

If your topology only consists of a single router with two VLANs, then DENSE mode would be preferable. Arguably, this is the simplest mechanism as it doesn’t require an RP (since you only have one router). In such a case, dense mode forwards multicast traffic on all interfaces until we are requested to stop forwarding. For more info on dense mode, take a look at the following:

I hope this has been helpful!