Multicast PIM Auto RP

Lessons Discussion
62

Hi Lagapides,

What if I wanted to get my device (listener) to igmp-join a specific group say 224.2.2.2, will auto-rp automatically detect ? Do I still need to configure the igmp join request on the closes router to the device?

63

Hello Champion

A rendezvous point (RP) performs very specific functions in a multicast network. One of those functions is to receive multicast requests from other multicast routers, which in turn receive join messages from hosts requesting particular multicast groups.

The Auto-RP feature simply automates the selection of the RP in a multicast topology. It does not affect the way that a particular host chooses to join a multicast group. Therefore, to answer your question directly, your listener must still issue the igmp-join command to join a particular multicast group. Does that make sense?

I hope this has been helpful!

Laz

64

I had some doubt that i guess could be related to the @sb6392 post.

I’ve done the following topology :

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R1 is the RP, R6 is the MA. R6’s directly connected PIM neighbors (R2, R5 & R7) and they are receiving the MA (224.0.1.40) msg telling them who is the RP (1.1.1.1) but as expected it’s not the same for R3, R4 & R8.

Then, i’ve issued the ip pim autorp listener only on R2 to test out if R2 forwards the MA 224.0.1.40 towards R3 and it worked as expected (R3 has the info of the RP 1.1.1.1)

Then, i’ve issued no ip pim autorp listener on R2, waited a couple of mins, and then tun0 gone down on R3 therefore it lost the info for the RP as expected.

Later, i’ve swaped R2 ↔ R3 wan ip add (R2 now has 192.168.23.3 and R3 192.168.23.2 making the R2 the PIM neigh DR for this 192.168.23.0/24 subnet) , and later R3 tun0 gone up and has again the info of RP 1.1.1.1 without needing the ip autorp listener on R2.

65

Hello Juan

Thanks for the detailed topology and test. It looks like you confirmed this behavior. Since R2 became the DR between R2 and R3, it was able to forward the mapping advertisements to R3 without implementing the ip autorp listener command on R2.

Thanks so much for sharing!

Laz

67

This is one of the best lessons on AutoRP and it’s intend. Thanks for sharing!

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