1

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?

2

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.

| improve this answer | |
  • 1
    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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.