Pim-Dm (Protocol Independent Multicast — Dense Mode) - Fujitsu PSWITCH 2048T User Manual

FUJITSU PSWITCH
The edge routers remove interfaces with no group members from their multicast
trees. If all the downstream interfaces are removed, the router sends a prune
message to its upstream neighbors. Every prune message has a lifetime, after
which the interface is joined back onto the delivery tree. If the unwanted
datagrams still appear, the prune is initiated again. If a host joins a previously
pruned branch of a tree, the DVMRP routers use graft messages to cancel the
previous prunes.
Limitations
- This software DVMRP does not support DVMRP tunnels.
- This software supports only the IPv4 version of DVMRP.
- This software DVMRP does not support DMVRP netmask capability.
3.1.3.8. PIM-DM (Protocol Independent Multicast — Dense Mode)
Protocol Independent Multicast (PIM) protocols are not dependent on any
particular unicast routing protocols to construct forwarding information for
multicast packets, although unicast information is needed for forwarding packets.
The Dense Mode (DM) version of PIM is most appropriate for networks with
relatively plentiful bandwidth and with at least one multicast member in each
subnet.
PIM-DM assumes that all hosts are part of a multicast group and forwards packets
to hosts until informed that group membership has changed. A group membership
change results in the multicast delivery tree being pruned.
The PIM-DM protocol operates as follows:
1. The first message for any (source, group) pair is forwarded to the entire
multicast network, with respect to the time-to-live (TTL) value in the packet.
2. TTL restricts the area flooded by the packet.
3. All leaf routers with no members in a directly attached subnet send prune
messages to the upstream router.
4. Any branch for which a prune message is received is deleted from the delivery
tree.
PIM-DM uses Reverse Path Forwarding (RPF), which is the fundamental concept in
multicast routing that enables routers to correctly forward multicast messages
down the distribution tree. RPF makes use of the existing unicast routing table to
determine the upstream and downstream neighbors and build a source-based
shortest-path distribution tree. A router forwards a multicast message only if the
multicast message is received on the upstream interface. This RPF check helps to
guarantee that the distribution tree is loop-free.
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User's Guide
December/2018