PIM Register – There are Clients, but no Source

In the last two articles in this series, I covered PIM register process in general, as well as how the process looks like when there is a source transmitting, but no interested clients. In this post, let’s explore how the process looks like when there are interested clients, but there is no source.To illustrate my point, I will use the same simple network diagram as I did in the previous two posts.

In the first part of this process, client will register. This procedure is very similar to the previous example when there was already a source transmitting. Let’s examine it on the diagram.

1. Client CLI wishes to join the group G and it sends IGMP membership report out its interface. This is learned by DR, which creates the (*, G) state
2. DR forwards the request as PIM join towards the RP for (*, G). All routers on the way to RP, including RP itself create appropriate (*, G) state.

This situation will remain as it is, as long as there are clients that have joined the group.

Let’s say that after a while, source begins to send the traffic towards group G. This is explained on Figure 3.

1. Source start transmitting. This creates both (*, G) and (S, G) states on first-hop-router, FHR.
2. Since the source is directly connected, FHR will start sending incoming multicast traffic as unicast to RP – register message. This creates (S, G) state on RP.
3. RP decapsulates incoming unicast traffic and sends it over existing (*, G) tree towards the client.
4. DR simply routes incoming multicast traffic to the client.

This is not all that happens, however. Almost simultaneously with step 3 above, RP is going to do something else. This is explained on Figure 4.

• Simultaneously with item 3 in the previous Figure, RP will send (S, G) PIM join towards the source.
• As soon as tree is built and FHR receives this join, it will start sending traffic received from the source as multicast over the new tree.
• Once the tree is built, FHR will stop sending the unicast register message.

Happy studies!

–
Marko Milivojevic – CCIE #18427
Senior Technical Instructor – IPexpert
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Tags: CCIE, CCIE Security, ccie security training, CCIE Service Provider, multicast, pim register