User's Manual

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Chapter 74 PIM-DM Configuration
74.1 PIM-DM Introduction
Protocol Independent Multicast Dense Mode (PIM-DM) is a multicast routing protocol in dense mode. By
default, when the multicast source starts to send the multicast data, all network nodes in the domain receive
the data. Therefore, PIM-DM forwards the multicast packets in broadcast-pruning mode. When the multicast
source starts to send data, the switches alongside forward the multicast packets to all PIM-activated interfaces
except the RPF interface. In this way, all network nodes in the PIM-DM domain can receive these multicast
packets. To finish the multicast forwarding, the switches alongside need create the corresponding multicast
routing item (S,G) for group G and its source S. The routing item (S,G) includes the multicast source address,
multicast group address, incoming interface, outgoing interface list, timer and logo.
If there is no multicast group member in a certain network segment, PIM-DM will send the pruning information,
prune the forwarding interface connecting the network segment and then establish the pruning state. The
pruning state corresponds to the timeout timer. When the timer times out, the pruning state turns to be the
forwarding state again and the multicast data can be forwarded along these branches. Additionally, the pruning
state contains information about the multicast source and the multicast group. When the multicast group
member appears in the pruning area, PIM-DM actively sends the graft message to the upper field without
waiting for the pruning state of the upper field to time out, turning the pruning state to the forwarding state.
As long as source S still sends information to group G, the first-hop switch will periodically send the refreshing
information of the routing item (S,G) to the nether original broadcast tree to finish refreshing. The state
refreshing mechanism of PIM-DM can refresh the state of the downstream, ensuring that the pruning of the
broadcast tree does not time out.
In the multi-access network, besides the DR selection, PIM-DM also introduces the following mechanisms:
 Use the assertion mechanism to select the unique forwarder to prevent the multicast packet
from being repeatedly forwarded.
 Use the add/prune restraint mechanism to reduce redundant add/prune information.
 Use the pruning deny mechanism to deny improper pruning actions.
In the PIM-DM domain, the switches that run PIM-DM periodically send the Hello information to
achieve the following purposes:
 Discover neighboring PIM switches.
 Judge leaf networks and leaf switches.
 Select the designated router (DR) in the multi-access network.
To be compatible with IGMP v1, PIM-DM is in charge of the DR choice. When all PIM neighboring routers on
the interface support DR Priority, the neighboring router with higher priority is selected as the DR. If the priority
is the same, the neighboring router with the maximum interface IP value is selected as the DR. If the priority
is not shown in the Hello message of multiple routers, the router whose interface has the biggest IP value is
selected as the DR.
The PIM-DM v2 of DBCOM’s switches supports the neighbor filtration list, CIDR, VLSM and IGMP
v1-v3.