Hello David
The term “IGMP Join” is an informal term used to refer to an IGMP Membership Report, so yes it is the same thing.
I hope this has been helpful!
Laz
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Clarification please.
So if you’re running PIM-Dense, it’s always going to be a Source Tree topology, and if it’s PIM-Sparse, it’s going to be in a Shared Tree topology with one or more RP’s. Am I understanding that correctly?
Thank you
Hello Terry
Yes, you have understood it correctly. Here’s a brief explanation for more clarity:
PIM-Dense Mode: In PIM-DM, the multicast traffic is initially flooded to all parts of the network (Source Tree topology). Non-wanting parts of the network then prune back the traffic. This is called a “push” model. PIM-DM primarily uses the source tree topology, more commonly called the Shortest Path Tree (SPT), with each source having a separate distribution tree.
PIM-Sparse Mode: PIM-SM initially sends no traffic until a receiver on a network segment indicates it is interested in receiving the traffic. The traffic is then sent to that network segment by the RP (Rendezvous Point) in a Shared Tree topology. This is called a “pull” model. PIM-SM typically uses the shared tree topology, most often called a Root Point Tree (RPT) initially. This delivers greater simplicity of implementation and lower overhead. When a host wants to receive the multicast traffic, it sends a join message upstream toward the RP. Once traffic for a group is flowing, if the volume of traffic and the number of receivers warrant it, PIM-SM can switch over to the source tree topology for greater efficiency.
I hope this has been helpful!
Laz
I know the router has to frame the multicast packet before it goes out of the interface. This leads to my question. What dest Mac address does the frame get? I am assuming its not ff-ff-ff-ff-ff-ff
cause that would likely not work with IGMP snooping. So the only logical thing I can think of is that the router has to calculate the Mac address associated with the specific multicast ip address.
That way once the frame is sent out the hosts that are interested in the multicast traffic get it and IGMP can filter out the traffic to host that dont want it.
Am i understanding this correctly?
I am going to assume the router knows it’s a multicast and calculates the corresponding Mac address that is tied to the multicast IP address for the frame.
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Hello Patrick
Your assumptions are correct. A router, when encapsulating a multicast IP packet into an Ethernet frame, it will use a well-defined mechanism of IP address to MAC address mapping to determine the correct destination MAC address. There are a series of MAC addresses that are reserved for use as multicast MAC addresses. The whole process is described in detail in the following lesson:
If you have further questions on this, feel free to ask!
I hope this has been helpful!
Laz
I was also confused when I saw ‘IGMP join’ and ‘PIM join’ messages depicted in the same illustration. I thought it was a typo, derived from the two messages: ‘PIM join’ and ‘IGMP membership report.’ However, it turns out that ‘IGMP join’ is an informal term.
I thought it might be worth adding a note in the following sentence: ‘H3 wants to receive this multicast traffic, so it sends an IGMP join message (also known as an ‘IGMP membership report’) for multicast group 239.1.1.1.’
Hello Zhassulan
Thanks for your suggestion. I will let Rene know to take a look and consider making the changes. Your feedback is appreciated!
Laz
Hello Team,
i am confused on IGMP join and PIM join. could you please explain bit on these ?
PIM Join - between multicast routers
IGMP Join - Between Host and Switch/Router
Hello Sathish
In the IGMP join process, the host sends an IGMP join message to its local multicast router, expressing its interest in a specific multicast group. The local router then updates its multicast routing table and begins forwarding the relevant multicast traffic to that host.
The PIM Join/Prune message is sent by a router upstream to the source or the rendezvous point (RP) to signal interest or disinterest in receiving multicast traffic. When a router has interested hosts downstream for a particular multicast group, it will send a PIM join message towards the source of the multicast group.
So IGMP join is used by hosts to inform the local multicast router of their desire to join a multicast group and start receiving traffic for that group. PIM join is used by routers to inform upstream routers that they want to receive multicast traffic from a particular group to server those multicast hosts.
I hope this has been helpful!
Laz