Similarities Between PIM-DM and PIM-SM
PIM-SM has many similarities to PIM-DM. Like PIM-DM, PIM-SM uses the unicast routing table
to perform RPF checks—regardless of what unicast routing protocol populated the table. (Like
PIM-DM, the “protocol independent” part of the PIM acronym comes from the fact that PIM-SM
is not dependent on any particular unicast IP routing protocol.) In addition, PIM-SM also uses the
following mechanisms that are used by PIM-DM:
■ PIM Neighbor discovery through exchange of Hello messages.
■ Recalculation of the RPF interface when the unicast routing table changes.
■ Election of a DR on a multiaccess network. The DR performs all IGMP processes when
IGMPv1 is in use on the network.
■ The use of Prune Overrides on multiaccess networks.
■ Use of Assert messages to elect a designated forwarder on a multiaccess network. The winner
of the Assert process is responsible for forwarding unicasts onto that subnet.
These mechanisms are described in the “Operation of Protocol Independent Multicast Dense
Mode” section and thus are not repeated in this section.
NOTE The preceding list was derived, with permission, from Routing TCP/IP, Volume II, by
Jeff Doyle and Jennifer DeHaven Carroll.