Bidirectional PIM
PIM-SM works calmly with a almost baby cardinal of multicast senders. However, in cases
with a ample cardinal of senders and receivers, PIM-SM becomes beneath efficient. Bidirectional PIM
addresses this about disability by hardly alteration the rules acclimated by PIM-SM.
To acknowledge bidirectional PIM, a abrupt analysis of PIM-SM’s accustomed operations is useful. While
many variations can occur, the afterward accepted accomplish can be acclimated by PIM-SM:
1. The RP builds a aggregate tree, with itself as the root, for forwarding multicast packets.
2. When a antecedent aboriginal sends multicasts, the router abutting the antecedent assiduously the multicasts to
the RP, encapsulated central a PIM Register message.
3. The RP joins the source-specific timberline for that antecedent by sending a PIM Accompany against that source.
4. Later, the routers absorbed to the aforementioned LANs as the receivers can accelerate a PIM Accompany against the
source to accompany the SPT for that source.
With bidirectional PIM, the aftermost three accomplish in this account are not performed. Bidirectional PIM instead
follows these steps:
1. As with accustomed PIM-SM, the RP builds a aggregate tree, with itself as the root, for forwarding
multicast packets.
2. When a antecedent sends multicasts, the router accepting those multicasts does not use a PIM
Register message. Instead, it assiduously the packets in the adverse administration of the aggregate tree,
back up the timberline against the RP. This action continues for all multicast packets from the source.
3. The RP assiduously the multicasts via the aggregate tree.
4. All packets are forwarded per Accomplish 2 and 3. The RP does not accompany the antecedent timberline for the
source, and the blade routers do not accompany the SPT, either.
The name “bidirectional” comes from Step 2, in which the router abreast the antecedent assiduously packets
back up the timberline against the RP. The added administration in the timberline is acclimated at Step 3, with the RP
forwarding multicasts application the aggregate tree.