Information About Optimizing PIM Sparse Mode in a Large IP
Multicast Deployment
PIM Registering Process
IP multicast sources do not use a signaling mechanism to announce their presence. Sources just send their
data into the attached network, as opposed to receivers that use Internet Group Management Protocol (IGMP)
to announce their presence. If a source sends traffic to a multicast group configured in PIM sparse mode
(PIM-SM), the Designated Router (DR) leading toward the source must inform the rendezvous point (RP)
about the presence of this source. If the RP has downstream receivers that want to receive the multicast traffic
(natively) from this source and has not joined the shortest path leading toward the source, then the DR must
send the traffic from the source to the RP. The PIM registering process, which is individually run for each (S,
G) entry, accomplishes these tasks between the DR and RP.
The registering process begins when a DR creates a new (S, G) state. The DR encapsulates all the data packets
that match the (S, G) state into PIM register messages and unicasts those register messages to the RP.
If an RP has downstream receivers that want to receive register messages from a new source, the RP can either
continue to receive the register messages through the DR or join the shortest path leading toward the source.
By default, the RP will join the shortest path, because delivery of native multicast traffic provides the highest
throughput. Upon receipt of the first packet that arrives natively through the shortest path, the RP will send
a register-stop message back to the DR. When the DR receives this register-stop message, it will stop sending
register messages to the RP.
If an RP has no downstream receivers that want to receive register messages from a new source, the RP will
not join the shortest path. Instead, the RP will immediately send a register-stop message back to the DR. When
the DR receives this register-stop message, it will stop sending register messages to the RP.
Once a routing entry is established for a source, a periodic reregistering takes place between the DR and RP.
One minute before the multicast routing table state times out, the DR will send one dataless register message
to the RP each second that the source is active until the DR receives a register-stop message from the RP.
This action restarts the timeout time of the multicast routing table entry, typically resulting in one reregistering
exchange every 2 minutes. Reregistering is necessary to maintain state, to recover from lost state, and to keep
track of sources on the RP. It will take place independently of the RP joining the shortest path.
PIM Version 1 Compatibility
If an RP is running PIM Version 1, it will not understand dataless register messages. In this case, the DR will
not send dataless register messages to the RP. Instead, approximately every 3 minutes after receipt of a
register-stop message from the RP, the DR encapsulates the incoming data packets from the source into register
messages and sends them to the RP. The DR continues to send register messages until it receives another
register-stop message from the RP. The same behavior occurs if the DR is running PIM Version 1.
When a DR running PIM Version 1 encapsulates data packets into register messages for a specific (S, G)
entry, the entry is process-switched, not fast-switched or hardware-switched. On platforms that support these
faster paths, the PIM registering process for an RP or DR running PIM Version 1 may lead to periodic
out-of-order packet delivery. For this reason, we recommend upgrading your network from PIM Version 1
to PIM Version 2.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
330 OL-32598-01
IP Multicast Optimization: Optimizing PIM Sparse Mode in a Large IP Multicast Deployment
Information About Optimizing PIM Sparse Mode in a Large IP Multicast Deployment
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