Cisco Cisco IOS Software Release 12.4(4)T

Users can translate externally received multicast destination addresses to addresses that conform to 
their company’s internal addressing policy.

Provides logical separation between private and public multicast networks.

Provides the flexibility to forward multicast packets—translate or untranslated—out the same 
outgoing interface.

Provides redundancy by allowing users to get identical feeds from two ingress points in the network 
and route them independently.

Users can use the subnet of their choice to be the source network and scope it appropriately.

A rendezvous point (RP) is a role that a router performs when operating in PIM-SM or bidirectional PIM. 
An RP is required only in networks running PIM-SM or bidirectional PIM. In PIM-SM, only network 
segments with active receivers that have explicitly requested multicast data will be forwarded the traffic.

An RP acts as the meeting place for sources and receivers of multicast data. In a PIM-SM network, 
sources must send their traffic to the RP. This traffic is then forwarded to receivers down a shared 
distribution tree. By default, when the first hop router of the receiver learns about the source, it will send 
a Join message directly to the source, creating a source-based distribution tree from the source to the 
receiver. This source tree does not include the RP unless the RP is located within the shortest path 
between the source and receiver.

In most cases, the placement of the RP in the network is not a complex decision. By default, the RP is 
needed only to start new sessions with sources and receivers. Consequently, the RP experiences little 
overhead from traffic flow or processing. In PIM version 2, the RP performs less processing than in PIM 
version 1 because sources must register only periodically with the RP to create state.

PIM sparse mode (PIM-SM) uses a pull model to deliver multicast traffic. Only network segments with 
active receivers that have explicitly requested the data will receive the traffic.

Unlike dense mode interfaces, sparse mode interfaces are added to the multicast routing table only when 
periodic join messages are received from downstream routers, or when a directly connected member is 
on the interface. When forwarding from a LAN, sparse mode operation occurs if an RP is known for the 
group. If so, the packets are encapsulated and sent toward the RP. When no RP is known, the packet is 
flooded in a dense mode fashion. If the multicast traffic from a specific source is sufficient, the first hop 
router of the receiver may send Join messages toward the source to build a source-based distribution tree.

PIM-SM distributes information about active sources by forwarding data packets on the shared tree. 
Because PIM-SM uses shared trees (at least, initially), it requires the use of a rendezvous point (RP). 
The RP must be administratively configured in the network. 

In sparse mode, a router assumes that other routers do not want to forward multicast packets for a group, 
unless there is an explicit request for the traffic. When hosts join a multicast group, the directly 
connected routers send PIM Join messages toward the RP. The RP tracks multicast groups. Hosts that 
send multicast packets are registered with the RP by the first hop router of that host. The RP then sends 
Join messages toward the source. At this point, packets are forwarded on a shared distribution tree. If 
the multicast traffic from a specific source is sufficient, the first hop router of the host may send Join 
messages toward the source to build a source-based distribution tree.