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DES-7200 Configuration Guide Chapter 5 PIM-DM
Configuration
5-1
5 PIM-DM Configuration
5.1 PIM-DM Overview
PIM-DM (Protocol Independent Multicast-Dense Mode), a multicast routing
protocol in dense mode, is suitable for the environments of small network size
and centralized multicast members. Since PIM-DM does not rely on any specific
unicast routing protocol, it is called protocol independent multicast routing
protocol. PIM-DM is defined in RFC 3973.
PIM-DM devices discover neighbors through Hello messages. After start, a
PIM-DM device sends a Hello message to the PIM-DM-enabled interface
periodically. The Hello message has a field called Hello Hold Time, which defines
the period that a neighbor waits for the next message. If the neighbor has not
received the next Hello message from the sender within this period, it announces
the device’s death.
PIM-DM sets up a multicast tree through flood and prune. Assume that when a
multicast source begins to send a multicast packet, all the systems in the network
need to receive this packet. As a result, this packet is forwarded to every system.
The reverse path forwarding check is done for the packets received from the
upstream interface. Those packets who fail to pass the check will be dropped. For
the packets passing the check, the egress is calculated based on the (S, G) pair
of the packets, or 从 source address and group address. If the egress exists, an
egress entry is set up fro the (S, G) pair and the multicast packet is forwarded
through this egress. If the calculated egress is null, a prune message is sent to
RPF, informing the upstream neighbor not to forward the multicast packets from
the (S, G) pair to this egress. Upon receiving the prune message, the upstream
interface marks the sending interface as pruned status, and set a pruned state
timer. In this way, a multicast forwarding tree at the root of multicast source is set
up.
PIM-DM utilizes the Assert mechanism to eliminate redundant routes.
Figure 1-4 Figure 4 PIM-DM’s Assert mechanism