Dell Force10 E300, Configuration Manual, Page: 749 / 1262

PIM Dense-Mode | 749
When a router receives a prune message, it flags the relevant (S,G) entry and sets a timer. If the timer
expires, it begins flooding traffic out of the interface, and downstream routers must again evaluate whether
to prune itself from the tree. To prevent the timer from expiring, while the source is sending traffic for the
(S,G), the first-hop router periodically sends an (S,G) state-refresh messages down the entire SPT. Router
that set the prune flag for the (S,G) entry reset the timer when they receive the message.
Requesting Multicast Traffic
When a new receiver joins a multicast group it sends an IGMP Membership Report to its gateway router.
The gateway router sends a PIM Graft message to its upstream neighbor, which sets a forwarding flag, and
propagates the graft message upstream, as shown in
Figure 33-3 . All remaining routers between the
receiver and the source also set a forwarding flag and propagate the graft message so that the receiver
begins receiving traffic for the (S,G).
Figure 33-3. Requesting Multicast Traffic in a PIM-DM Network
PIM-DM 003
Source
Receiver
R1
R2
R3
R4
Group Address: 239.192.0.1
Graft
Receiver