PIM Overview

In this post we will try to have an overview of PIM

PIM (Protocol Independent Multicast) is a protocol to carry multicast traffic, as the name indicates (Protocol Independent) PIM doesn’t have topology discovery built-in mechanisms, PIM relies on the unicast routing protocol to build the Multicast trees.

PIM comes in 4 modes

PIM-DENSE: implicit join, i.e the multicast traffic is flooded everywhere and then pruned from receivers that don’t want to receive this specific feed

PIM-SPARSE: explicit join, senders register with RP (Rendvous Point) and receivers contact RP routers to receive specific multicast feeds when needed

PIM-BiDirectional: doesn’t build SPT (shortest Path Tree) and doesn’t maintain (S,G) entries

PIM-Source Specific Multicast:

Source Specific Multicast method allows clients to choose the source of multicast feed. It requires IGMP version 3 though. SSM builds SPT tree and does not require RP to operate.

PIM maintains neighbour ship relation using hello messages (sent to 224.0.0.13), the default hello timer in Cisco routers is 30 seconds and hold down time is 3.5 the hello interval i.e 1min 45 seconds.

The Hello interval can be modified using the ip pim query under the interface level

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