PIM Sparse Mode

PIM sparse mode works the opposite way of dense mode. PIM dense mode assumes that all the multicast interfaces are interested in multicast packets, unless being told otherwise . In PIM sparse mode, the router assumes that none of the multicast inter-faces is interested in receiving multicast packets, unless a PIM join message is received on the interface. PIM sparse mode is more scalable than PIM dense mode, but the concept is more complex. PIM sparse mode uses the concept of a rendezvous point (RP). The RP is where the sender and the receivers meet first before the shortest-path tree is established. The shortest-path tree is the shortest path between the multi-cast sender and the receiver. For a particular multicast group , only one RP is chosen . The selection of the RP is done by either static configuration or dynamically learned through the Auto-RP mechanism.

PIM sparse mode discovers its neighbor the same way that PIM dense mode works. The PIM routers send out PIM query packets to discover PIM neighbors on the link. In PIM sparse mode, the highest IP address on a LAN segment is elected the designated router. This desig-nated router is used to send PIM joins to the RP for the segment.

In sparse mode, multicast flow has two parts :

1. Receivers send PIM joins to the RP.

2. The source sends PIM registers to the RP.

In the PIM sparse mode join mechanism, the router that is closest to the receiving station sends the PIM join message to the RP. If more than one router exists on the LAN segment, the PIM DR sends the join to the RP. The PIM joins are then sent hop by hop toward the RP. Figure 12-8 illustrates the PIM sparse mode join mechanism.

Figure 12-8. PIM Sparse Mode Joining

In Figure 12-8, the PC sends IGMP joins to its Ethernet interface. Router R2 is the DR because it has a higher IP address on interface E1. Router R2 sends PIM joins toward the RP, so R2 sends PIM joins to Router R1. Router R1 sends the PIM joins toward the RP. Therefore, the PIM joins are sent hop by hop from the leaf router closest to the receiver, all the way to the RP. The leaf router is defined as the outermost edge router that has only receivers connected.

The second part of the sparse mode operation is the register process, which can be dissected into the following sequence of events:

1. The sparse mode register messages are sent by the router that is closest to the sender of the multicast stream. If more than one router exists on the LAN segment, the PIM DR is responsible for sending the PIM register message to the RP.

2. When the sender begins sourcing a multicast stream, the first-hop router encapsulates the multicast packets into the PIM register messages and unicasts them to the RP.

3. When the RP receives the register message, it first de-encapsulates the multicast packet that it contains.

4. The RP then forwards the multicast packet toward any existing receivers and also sends a PIM join message toward the multicast source.

5. When the PIM join reaches the first-hop router to the source, the first-hop router starts forwarding native multicast traffic toward the RP, while still sending PIM register messages to the RP.

6. When the RP receives two copies of a multicast packet, one from the register message and the other from the native multicast path, the RP sends a register stop message to the first-hop router.

7. When the first-hop router receives the register stop message, it stops encapsulating the multicast traffic into the register message and forwards it natively to the RP.

Figure 12-9 illustrates the first part of the sparse mode register process, which corresponds to events 1 through 4 in the previous discussion. (The PC sources the multicast stream, and Router R1 encapsulates the multicast packets into PIM register messages and unicasts the packets to the RP.) When the RP receives the register packets, it de-encapsulates the multicast packets and forwards them downstream to the receivers. At the same time, the RP sends PIM joins toward the source. In this case, the RP sends joins to Router R2, and Router R2 sends PIM joins to R1.

Figure 12-9. PIM Sparse Mode Register Process, Part 1

Figure 12-10 is the continuation of the PIM sparse mode register process. When Router R1 receives the join from the RP, R1 begins to send the multicast traffic toward the RP. The register messages from R1 are still forwarded. The RP then receives two copies of the multicast packet, one from the register message and the other from the native multi-cast flow. When the RP sees two copies of the multicast packet, it sends a register stop message to the first-hop router ‚ in this case, Router R1. When R1 receives the register stop message, it stops encapsulating the multicast packets into register messages, and the traffic now flows natively from R1 to R2 and then to the RP. The RP then forwards the multicast stream to its downstream neighbors.

Figure 12-10. PIM Sparse Mode Register Process, Part 2

The sparse-mode pruning mechanism is the same as the one used in dense mode. If the router is not interested in the multicast traffic, it sends a PIM prune message to the upstream neighbor toward the source. For a more detailed description on PIM operation, refer to Beau Williamson's book Developing IP Multicast Networks, Volume 1.