Ericsson ECN330, User Manual

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Configuring the ECN330-switch
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1553-KDU 137 365 Uen D 2006-06-16
6.21.4 Configuring PIM-DM
Protocol-Independent Multicasting (PIM) provides two different modes of
operation: sparse mode and dense mode. Sparse mode (SM) is designed for
networks where the probability of multicast group members is low, such as the
Internet. Dense mode (DM), on the other hand, is designed for networks where
the probability of multicast group members is high, such as a local network. For
information on configuring PIM-SM, see “Configuring PIM-SM” on page 490.
PIM-DM is a simple multicast routing protocol that uses flood and prune to build
a source-routed multicast delivery tree for each multicast source-group pair. It is
simpler than DVMRP because it does not maintain it’s own routing table.
Instead, it uses the routing table provided by the unicast routing protocol
enabled on the router interface. When the router receives a multicast packet for
a source-group pair, PIM-DM checks the unicast routing table on the inbound
interface to determine if this is the same interface used for routing unicast
packets to the multicast source network. If it is not, the router drops the packet
and sends a prune message back out the source interface. If it is the same
interface used by the unicast protocol, then the router forwards a copy of the
packet to all the other interfaces for which is has not already received a prune
message for this specific source-group pair.
DVMRP holds the prune state for about two hours, while PIM-DM holds it for only
about three minutes. This results in more flooding than encountered with
DVMRP, but this is the only major trade-off for the lower processing overhead
and simplicity of configuration for PIM-DM.
6.21.4.1 Configuring Global PIM-DM Settings
PIM-DM is used to route multicast traffic to nodes which have requested a
specific multicast service through IGMP. It uses the router’s unicast routing table
to determine if the interface through which a packet is received provides the
shortest path back to the source. This is done on a per hop basis back toward
the source of the multicast delivery tree. PIM-DM uses three different techniques
to dynamically reconfigure the multicast spanning tree: broadcasting, pruning,
and grafting.
To use PIM-DM, it must be enabled globally for the router, as described below,
and for each interface that will support multicast routing as described in the next
section. Also note that IGMP must be enabled to allow the router to determine
the location of group members.