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I was going through the use cases of GRE tunnel and see that it is also used to carry multicast traffic over the internet.I understand that multicast IP belongs to class D address which is a public IP and can be routed.

Q1:Why is multicast not supported over the internet ? Why do we need to encapsulate in GRE ? Can you please explain in laymen terms ?

Q2:When we use the term 'internet', what exactly does it refer to? Would it be collection of all ISPs across the world?

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A1: Because normally you need to run PIM protocol in order to achieve end-to-end connectivity for multicast network. PIM doesn't normally run between ISP boundaries. It would be pretty complicated and protocol itself wasn't designed for that scale.

A2: It depends. From networking point of view I would say that Internet is merely a collection of public routes, i. e. BGP full view.

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    A1 isn't totally right - MBGP has existed for a long time as a mechanism to carry RPF information within a BGP address family and was introduced as a way to maintain independent multicast domains. It absolutely can- and does- scale pretty well. The issues at hand for why multicast isn't supported have a lot more to do with the cost and complexity of maintaining the infrastructure versus the potential revenue realized for doing so. – rnxrx Jan 8 '17 at 1:09
  • Yeah, but MBGP just distributes routing information. Without PIM forwarding will not work. It in turn implies every and each ISP should run PIM inside their network. – ar_ Jan 8 '17 at 12:20

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