Router instance in EC2 never receives IGMP join messages from other EC2 instances

Question

Within a single VPC with transit Gateway multicast I can succesfully pass multicast between instances and see IGMP joins in the TGW menus.
Now I want to extend to on prem, but it seems the router instance never receives join messages from instances within the VPC.
My software router in EC2 has pim neighbourship with my on prem router via GRE trunnel, but no joins from other EC2 instances.

Is there a way to tell the TGW to forward igmp join messages to the software router instance?

I worked around this static joins and an IGMP proxy, but it's rather kludgey.

I confirmed with wireshark my EC2 instance was sending IGMPV2 join messages, and they are reflected in the TGW console, but not received at the soft. router.

Brettski · Accepted Answer

VPC is an overlay network and it emulates Ethernet for you so that most regular operations work; but many things don't. For example it looks like broadcasts work (ARP, for example) but again those are emulated rather than actually happening. See [this YouTube video](https://youtu.be/8gc2DgBqo9U) for a lot more on how VPC works.

Same goes for multicast - in this case, Transit Gateway is acting as the multicast "hub" but it only implements some of the multicast functions - and it won't send the join messages back to your router - that's why you can see them being sent; why they are received by Transit Gateway; and why you're not seeing them being received again.

I'd encourage you to reach out to your local AWS Solutions Architect who can get you in touch with a networking specialist to discuss your use case.

Router instance in EC2 never receives IGMP join messages from other EC2 instances

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