In this section, we will discuss about Multicast – RPF check. RPF means Reverse Path forwarding.
To understand RPF check, first need to understand Unicast route path forwarding. In Unicast routing, The packet is routed sending “towards to destination” by using routing table.
But in Multicasting, the multicast packet routed by sending “ away from Source”. As you read this, both seem to be same. But it is not same. RPF check means, Every time router check that Multicast packet is going away from source and it should go away from source rather than to go towards destination. As discussed in last post, Multicast distribution Trees. The multicast source tree and shared tree. Multicast source will sit on top of Tree and then the multicast information is flooded. But this time router will ensure that Multicast packet is going away from source.
In Multicast routing as the packet is arrived, Router will check the interface and mark as upstream interface. That means from this interface Multicast packet arrived. Router will make sure that it will not go out from that interface. It will send from other interface, which will be downstream interface. This is Loop prevention mechanism for Multicast Routing.
RPF means reverse path forwarding; It has to make sure that multicast packet is going away from source. RPF will check every incoming multicast packet in router. It will ensure, multicast packet should go out of downstream interface.
RPF check depends on our Unicast routing table, So we need to make sure that our unicast routing is properly configured. Same time in case of troubleshooting, Plz check the topology and check one by one and make sure the packet is going away from source.
Those packets which don’t pass RPF check. It will get dropped.
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