A Host Joins A Group After Routers Have Already Switched To An Sp Tree - HP 7102dl - ProCurve Secure Router Configuration Manual

Configuring Multicast Support with PIM-SM
Overview
13-16
2. Although the RP creates the (S, G) entry, because the entry's outgoing
interface list is null, the RP does not send a join for the SP tree. The RP
also sends a register-stop to the source's DR.
3. The DR stops sending the encapsulated multicasts. It also drops all
multicasts because it has not received any joins for its SP tree.
4. When a host does join the multicast group, the RP can activate the SP tree
as soon as it receives the join.
Routers between the host and the RP build the shared tree by forwarding
joins towards the RP. The RP simply adds the interface on which it
receives the IGMP or PIM (*, G) join to both the (*, G) and the (S, G) entries.
It then sends an SP tree join towards the source. The DR receives the join
and begins forwarding multicasts instead of dropping them.
5. The edge router for the host can switch to the SP tree as described in
"Edge Routers" on page 13-12.
A Host Joins a Group After Routers Have Already Switched
to an SP Tree
A host can, of course, begin receiving multicasts immediately after it joins a
group if other hosts in its subnet are also members of the group. However,
PIM-SM forwards multicasts conservatively. If a subnet does not have any
members in a group, then the subnet will not receive multicast traffic for that
group. If a host does join the group, the traffic must be drawn down from the
source to the subnet.
Typically, when a router receives an IGMP join for a new multicast group, it
joins the corresponding RP tree. This could cause problems when a subnet
requests traffic for a group for which the DR is now forwarding traffic on an
SP tree.
PIM-SM addresses this issue by always forcing a router to copy the outgoing
interface list of a (*, G) entry to an (S, G) entry. When the router receives the
IGMP join, it adds the receiving interface to both entries.
In short, as soon as at least one host in subnet joins a group, it can begin
receiving multicast traffic on any active SP tree for that group.