Information Technology Reference
In-Depth Information
PIM has two modes of operation, Dense Mode and Sparse Mode. PIM Dense Mode is based
on a flood-and-prune methodology, whereby IP multicast traffic is flooded out all PIM
adjacencies, and unwanted streams are pruned. PIM Sparse Mode is based on an explicit
join methodology, whereby IP multicast traffic is forwarded down an adjacency only when
it has been explicitly requested.
On each router, an MDT is composed of an Incoming Interface (IIF) and an Outgoing Inter-
face List (OIL). The IIF is based on the Reverse Path Forwarding (RPF) information for the
particular (*,G) or (S,G). The source address of arriving IP multicast packets is used for the
RPF check, which is a paradigm shift from standard unicast routing. The RPF information
for an (S,G) indicates which interface is used to send traffic to the source via the shortest
path. This information is typically derived from the IGP. The RPF information for a (*,G)
uses the address of the Rendezvous Point (RP) instead of the source address. The RP is the
root of the shared tree in PIM-SM. Rendezvous points are covered in further detail in the
“Sparse Mode” section. Figure 11-6 shows an example of RPF.
Figure 11-6
Multicast Reverse Path Forwarding
Stream 1
Routing Table - R1
Prefix Next-hop
10.1.1.0 S0
(10.1.1.1, 224.1.1.1)
R2
(10.1.1.1, 224.1.1.1)
Stream 1
Stream 2
(10.1.1.1, 224.1.1.1)
S1
S0
R1
Figure 11-6 shows two copies of the same (S,G) arriving at R1. The first stream, Stream 1,
arrives on port S1 on R1. The source is 10.1.1.1. The routing table on R1 says the next hop
to 10.1.1.1 is through port S0. The stream arriving on port S1 fails the RPF check because
it arrives on a different interface than the path back to the source. Stream 2 arrives on port
S0 and passes the RPF check. R1 prunes Stream 1 and forwards Stream 2 to the locally
connected receiver.
