Multicast PIM Designated Router

This topic is to discuss the following lesson:

Thanks Rene, a very useful tutorial on ip pim, espescially on Designated Router, how a receiver joins a group and how the Rendezvous Point is mapped to the group. Fundamentals of ip pim sparse mode which i am working with. Thank you.

You are welcome Kevin, glad to hear you like it.

Thank You…

Very nice explanation - appreciated.

Too good. it is very crisp and clear. Answered to the point.
Too good. Keep posting similar posts for other multicast technologies as well.

Thanks,

Rene,
Awesome post, Thanks for the clear explanation!

  1. If I understand correctly, DR is per each Link (segment)?
  2. “This new DR already has the IGMP state for the required multicast groups because it also heard the IGMP membership reports from receivers on the segment. The only thing the new DR has to do is send a PIM join to the RP and our traffic flow will continue.”

one query. Why will new DR maintains IGMP state?-- Only DR has to maintain this, right?
Would it NOT Prune PIM (*, G) and (S, G) States when the DR fail over happened? Will there be a traffic loss until a new DR is elected?

  1. DR is only present in PIM-SM? No DR in PIM-SSM& DM, right?

Thank you, it’s explained very well :smiley:

Thank you. Awesome explanation and very structured. Waiting for PIM-SSM…

Great Post Rene,
Have a question.
Your statement here “As you can see R2,R3 and R4 are connected to the same multi-access network (switch). When R4 sends a PIM join message both R2 and R3 would receive it and forward it to R1.”

- Wouldnt R4 send an IGMP join since it doesnt have PIM enabled?
- What is the source address for that Join? If both R2 and R3 receive the join does that mean its sent using the unicast address of R4 and the destination of 224.0.0.13? Just need clarification on that point.

Thanks !

Hi Michael,

IGMP join is normally used by a end device, I’ve used a router but imagine that R4 is a computer or something. It doesn’t need PIM.

The source address will be 192.168.234.4 with destination 239.1.1.1 (the group you want to join). The 224.0.0.13 multicast address is used between PIM routers.

Here’s a wireshark capture of a IGMP join:

https://www.cloudshark.org/captures/015f44080e71

Rene

Got it. So what would be the source address of the PIM Join as it makes it way to the RP?

The source IP address of the PIM join will be the interface that is closest to the RP, its destination will be 224.0.0.13.

It all makes sense now…Multicast isnt that hard once you put it into small pieces. Thanks Rene!!!

Hi Rene,
In the same topology, if we consider R4 as a PIM router, which is also a DR for the LAN segment connecting R2 and R3 and if there is PIM or IGMP join coming from downstream connected to R4:

Does R4 send PIM joins upstream because it is a DR? what if R4 is non-DR in the LAN segment?

If, R4 is the DR, and it sends PIM join upstream, how will R2 and R3 process the joins, since none of them are DRs?

Thanks in advance
Vishal

Hi Vishal,

It is no problem if R4 is the DR on the 192.168.234.0/24 segment. It will be responsible for creating the PIM join, which will be forwarded upstream. R2 or R3 will forward it further upstream.

Rene

Hi Rene,

I have two questions :-

1- you said in your answer to Vishal that “R2 or R3 will forward it further upstream”, my question is which one exactly will forward it and depending on what this router is chosen instead of the other one ??

2- Making the priority of the DR 0 mean this router never elected as DR just like in ospf DR election or it mean something else ??

Hello Hussien

Which router will be chosen depends on what is called the PIM Assert mechanism. You can find detailed information about this at the following lesson:

When you set the priority of a router to 0, this means that it will never become a DR.

I hope this has been helpful!

Laz

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Thanks @lagapides for your answer,

I know how PIM assert working and it’s to elect one PIM forwarder downstream to the receiver but my question was about PIM join messages, which R4 will forward it upstream to R1 (RP), in this case the process reversed !! is PIM assert working in both cases or there is something else are making the decision ??

Hi Hussien,

It’s the job of the DR (Designated Router) to forward the PIM join upstream.

Rene

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