Is IGMP used in IPv4 with hosts sending data to multicast groups?

[Sam Brynes]

A lot of the documentation on multicast and PIM is focused on the receiver side receiving data. If a host wants to send data to a multicast group, is IGMP involved in any way? Or does it simply just source a packet from its interface IP address, destined to the multicast group address, and send it to the corresponding MAC address of the multicast group?

[Muhammad Awais Khan]

Hi,

My understanding for Multicast is that IGMP will be used by receivers , they send IGMP packets over network segment IGMP Membership message with a group address they want to join. 

PIM makes the multicast tree and path over PIM enabled Rotuers between source and receivers. Your understanding for Multicast source is correct that source is sending stream to the multicast group address. If any host want to join that stream or group, request will reach to PIM enable Routers using IGMP and then whole path will be formed between source and receivers.

[Vishnu Vardhan S]

Normally in Multicast we have data flowing from Source to the Hosts.

Your question is about the hosts sending a data to source if am not wrong. 

Usually the IGMP is used by the host to send Membership towards the source to inform it that it wants to join the multicast group.

am quite not sure what data host want to send ??

[Seb Rupik]

Hi there,

A multicast source sends its stream to the Designated Router on the segment it is connected to. This router is the FHR (First Hop Router). The source packets will be either PIM encapsulated and depending on the PIM mode (sparse or dense), sent towards the RP or flooded to adjacent routers. There is no IGMP involved.

The receiver sends IGMP messages to the LHR (Last Hop Router) which depending on the PIM mode either sent towards the RP or consumed by the LHR.

I attend this presentation where the presenter gives an enthusiastic talk about the entire process:

https://www.ciscolive.com/global/on-demand-library.html?search.event=ciscoliveanz2019&search=multicast#/session/15341979774660017eR6

cheers,

Seb.