HP 3500YL - Manual (Page 115)

PIM-SM (Sparse Mode)
PIM-SM Operation and Router Types
page 4-67.) When completed, the switchover from the RPT to a shorter SPT
can reduce unnecessary traffic concentrations in the network and reduce
multicast traffic throughput delays.
Note that the switchover from RPT to SPT is not instantaneous. For a short
period, packets for a given multicast group may be received from both the
RPT and the SPT. Also, in some topologies, the RPT and the SPT to the same
edge router may be identical.
PIM-SM
Router “A”
PIM-SM
Router “B”
PIM-SM
Router “C”
PIM-SM
Router “D”
Source of
Multicast
Group “X”
Host “Y”
SPT
Path Designated Router
(DR) for Unicast Source of
Multicast Group “X”
Elected Bootstrap Router
for the Domain, and
Elected Rendezvous
Point (RP) for Supporting
Multicast Group “X”
In default PIM-SM operation, the STP path
activates and the RPT path drops off after
the first multicast packet for a group is
received via the Rendezvous Point (RP).
Intermediate
Router in
RPT Path for
Group “X”
Figure 4-2. Example PIM-SM Domain with SPT Active To Support a Host that Has
Joined a Multicast Group
Restricting Multicast Traffic to Rendezvous-Point
Trees (RPTs)
An alternate method to allowing the domain to use SPTs is to configure all of
the routers in the domain to use only RPTs. However, doing so can increase
the traffic load in the network and cause delays in packet delivery.
Maintaining an Active Route for Multicast Group Members
The edge router itself and any intervening routers on the active tree between
the members (receivers) of a multicast group and the DR for that group, send
periodic joins. This keeps the active route available for as long as there is a
multicast receiver requesting the group. When a route times out or is pruned,
the DR ceases to send the requested group traffic on that route.
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