Protocol Independent Multicast or PIM dense mode (PIM-DM) is a push model where we flood all interfaces with multicast packets until a downstream router requests the upstream router to stop forwarding multicast traffic. This IP multicast routing protocol mode is effective in specific deployments with active multicast receivers on every subnet in the network. On the other hand, PIM sparse mode is a pull model where multicast traffic is sent only when requested.
How Does Multicast PIM Dense Mode (PIM-DM) Work?
In our example topology below, we have a server that sends a multicast packet towards R1. Once R1 receives this multicast traffic, it will create an entry in its multicast routing table where it stores the source IP address and multicast group address. It will then send the traffic on all of its interfaces except the one where it received the traffic.
- Upstream Router — The router from which we receive multicast traffic.
- Downstream Router — The router to which we send multicast traffic.
Each downstream router that is not interested in the multicast traffic will send a prune message to its upstream router. Prune messages tell the source routers that it doesn’t need that multicast traffic right now.
PIM Dense Mode Pruning
Prunes expire after three minutes in PIM-DM. This allows multicast traffic to be reflooded to all routers in the same manner as it did during the initial flood. This periodic flood of every 3 minutes and prune behavior is typical and must be considered when designing a network to employ PIM-DM.
Below are the reasons why a router will send a prune message:
1. No directly connected host that is interested in the multicast traffic.
2. No downstream router that is interested in the multicast traffic.
3. If multicast traffic was received on a non-RPF interface.
PIM-DM is appropriate for small networks with active receivers on each subnet. PIM-DM is not typically suggested for production environments. Nevertheless, it can be beneficial in a lab environment because it is simple to set up.
Reverse Path Forwarding
In unicast routing, we only look for the destination IP address and find a way to get there. In multicast routing, the source IP address is important. PIM will only accept multicast packets on an interface that are used to reach the source. Reverse Path Forwarding or RPF failure is when we drop multicast packets that we receive on an interface that we don’t use to reach the source.
RPF check procedure works as follows:
- The router searches the unicast routing table for the source IP address. It checks if the packet has arrived on the interface on the return path back to the source.
- If yes, the RPF check succeeds. The packet will now be forwarded out to the interfaces present in the outgoing interface list of a multicast routing table entry.
- If not, the multicast packet is dropped.
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