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FUJITSU PSWITCH
User’s Guide
96
December/2018
sending register messages to the RP for sources.
PIM-SM uses shared trees by default and implements source-based trees for
efficiency. PIM-SM assumes that none of the hosts want multicast traffic unless
they specifically ask for it. It creates a shared distribution tree centered on a
defined RP from which source traffic is relayed to the receivers. Senders first send
the multicast data to the RP, which, in turn, sends the data down the shared tree to
the receivers. Shared trees centered on an RP do not necessarily provide the
shortest or optimal path. In such cases, PIM-SM provides a means to switch to more
efficient source-specific trees.
As soon as an active source sends a packet to its DR, the DR is responsible for
registering the source with the RP and requesting that the RP build a tree back to
that DR. The DR encapsulates the multicast data from the source in a Register
message and unicasts that data to the RP.
PIM-SM uses the explicit join model whereby hosts wishing to receive multicast
packets send PIM join messages to the RP using the DR. When a DR gets a
membership indication for a new group, it looks up the RP associated with the
group and sends a join message to the RP.
Upon reception of the first data packet, the DR at the receiver's end automatically
switches to the source's shortest path tree by initiating a PIM Join towards the
source. After sending the PIM Join (as said above), the DR also prunes the RP tree
by initiating a PIM Prune message towards the RP. This ensures that the multicast
data flows only on the source's shortest path tree and does not flow down the
shared RP tree.
An RP that is configured as Candidate (C-RP) for a particular multicast group range
sends periodic C-RP advertisements to the BSR. PIM-SM allows only one interface to
be configured as C-RP on a router to elect RP. It supports up to a maximum of 20
C-RPs for an interface.
The PIM-SM feature supports distributing both IPv4 and IPv6 routes. For
information about the PIM-SM Bootstrap Router (BSR) mechanism, see
draft-ietf-pim-sm-bsr-05.
3.1.3.9.1.
PIM Hello Interval Tuning
The ip pim hello-interval command controls the interval that a PIM hello packet is
transmitted out each PIM-enabled interface.
The PIM hello packets are used to discover PIM neighbors and to determine the
Designated Router (DR) on each network segment. The default interval for the PIM
hello packets to be sent is 30 seconds. A PIM neighbor is considered down after