Virtual Router to Support Multicast

##### ISSUE TYPE

* Improvement Request

##### COMPONENT NAME

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Virtual Router
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##### CLOUDSTACK VERSION

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4.19.1.1
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##### CONFIGURATION

##### OS / ENVIRONMENT

##### SUMMARY

Currently Cloudstack Virtual Router doesnt support Multicast from external networks (eg. Private Gateway).

In our case, our customers require multicast support because they decode Stock Market Feeds from the Stock Exchange. And Majority Stock Exchange distributes market feeds in ITCH format, which is Multicast.

In order to overcome this limitation now, we had to resort to VNF + L2 Networks.

I also have a curious question, if Cloudstack VR does not support multicast, then why is there a UDP Protocol option in the Network ACL Rules?

##### STEPS TO REPRODUCE

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##### EXPECTED RESULTS

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For VR to support multicast natively
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##### ACTUAL RESULTS

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VR does not support multicast at the moment.
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该 issue 指出了 CloudStack 的 Virtual Router，并请求为外部网络提供原生多播支持，但没有指定任何文件、测试、命令或入口点。首先定位 Virtual Router 的网络实现以及现有的协议或路由测试。对于所描述的外部网络用例，当多播流量能够通过 Virtual Router 原生工作时，即表示完成。

由索引模型根据 Issue 内容生成。