Install guide

6 Software Reference
Protocol Independent Multicast (PIM)
The two Protocol Independent Multicast routing protocols rely on the presence
of an existing unicast routing protocol to adapt to topology changes, but are
independent of the mechanisms of the specific unicast routing protocol.
PIM Sparse Mode and PIM Dense Mode must be enabled with a special feature
licence. To obtain one, contact an Allied Telesis authorised distributor or
reseller.
The switch can be configured as a Multicast Border Router, with different
interfaces connecting to multicast domains that use different multicast routing
protocols. Therefore, some PIM interfaces can be configured for PIM-SM and
others for PIM-DM. Multicast packets are forwarded between the Sparse Mode
and Dense Mode domains as required.
PIM Dense Mode
Unlike PIM Sparse Mode, PIM Dense Mode (PIM-DM) does not use a
designated router, bootstrap router, or rendezvous points.
PIM-DM employs the Reverse Path Multicasting (RPM) algorithm. When
operating:
■ PIM-DM relies on the presence of an existing unicast routing protocol to
provide routing table information to build up information for the multicast
forwarding database, but it is independent of the mechanisms of the
specific unicast routing protocol.
■ PIM-DM forwards multicast traffic on all downstream interfaces until
explicit prune (un-join) messages are received. PIM-DM is willing to accept
the overhead of broadcast-and-prune in the interests of simplicity and
flexibility, and of eliminating routing protocol dependencies.
PIM-DM assumes that when a source starts sending, all downstream systems
want to receive multicast datagrams. Initially, multicast datagrams are flooded
to all areas of the network. If some areas of the network do not have group
members, dense-mode PIM prunes the forwarding branch by setting up prune
state. The prune state has an associated timer, which on expiration turns into
forward state, allowing data to go down the branch that was previously in
prune state.
The prune state contains source and group address information. When a new
member appears in a pruned area, a router can “graft” toward the source for
the group, turning the pruned branch into a forwarding branch. The
forwarding branches form a tree rooted at the source leading to all members of
the group. This tree is called a source rooted tree.
The broadcast of datagrams followed by pruning of unwanted branches is
often referred to as a broadcast-and-prune cycle, typical of dense mode
Mode Description
PIM Dense Mode Suitable for networks where bandwidth is plentiful, and where
members of a multicast group are densely distributed on the
network.
PIM Sparse Mode Suitable when members of the multicast groups are more
sparsely distributed over the network because it results in less
duplication of data packets over the network.