TP-Link T3700G-28TQ T3700G-28TQ V1 UG - Page 237

PIM Modes, PIM DM, PIM SM: Protocol Independent Multicast-Sparse Mode

Page 237 highlights

(1) If the corresponding entry (S, G) exists and the packet's actual arriving interface is the same as the input interface in the multicast forwarding table, the packet will be forwarded to all the output interfaces. (2) If the corresponding entry (S, G) exists and the packet's actual arriving interface is different from the input interface in the multicast forwarding table, the router will perform RPF check on this packet:  If the check result shows that the RPF interface is the same as the input interface in the current (S, G) entry, which indicates that the (S, G) entry is correct and the packet from the wrong path will be discarded;  If the check result shows that the RPF interface is the different from the input interface in the current (S, G) entry, which indicates that the (S, G) entry is invalid and the router will correct the input interface to the packet's actual arriving interface, and forward this packet to all the output interfaces. (3) If the corresponding entry (S, G) doesn't exist, the router will still perform the RPF check on this multicast packet. With the RPF interface as the input interface, the router will create corresponding entry with the RPF interface as the input interface combining related routing information, and send this entry to the multicast forwarding table:  If the packet's actual arriving interface is exactly the RPF interface, the RPF check will pass and the packet will be forwarded to all the output interfaces;  If the packet's actual arriving interface is not the RPF interface, the RPF check fails and this packet will be discarded.  PIM Modes PIM can be divided into two modes according to different routing mechanisms:  PIM DM: Protocol Independent Multicast-Dense Mode  PIM SM: Protocol Independent Multicast-Sparse Mode  PIM DM PIM DM is a multicast routing protocol in dense mode. It uses Push Mode to transfer multicast packets and applies to small network with relatively dense multicast group members. The working mechanism of PIM DM is illustrated as below:  PIM DM assumes that there is at least one multicast group member in each subnet of the network, and the multicast packets will be flooded to all the nodes in the network. Then branches without receivers are pruned from the distribution tree, leaving only branches that contain receivers. This "flood-and-prune" process takes place periodically. The pruned branches can also resume to forwarding state periodically.  When a new receiver on a previously pruned branch of the tree joins a multicast group, the PIM DM takes the Graft (see Grafting) mechanism to actively resume this node's function of forwarding multicast data, thus reducing the time it takes to resume to the forwarding state. 226

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(1)
If the corresponding entry (S, G) exists and the packet’s actual arriving interface is the same
as the input interface in the multicast forwarding table, the packet will be forwarded to all the
output interfaces.
(2) If the corresponding entry (S, G) exists and the packet’s actual arriving interface is different
from the input interface in the multicast forwarding table, the router will perform RPF check on
this packet:
If the check result shows that the RPF interface is the same as the input interface in the current
(S, G) entry, which indicates that the (S, G) entry is correct and the packet from the wrong
path will be discarded;
If the check result shows that the RPF interface is the different from the input interface in the
current (S, G) entry, which indicates that the (S, G) entry is invalid and the router will correct
the input interface to the packet’s actual arriving interface, and forward this packet to all the
output interfaces.
(3)
If the corresponding entry (S, G) doesn’t exist, the router will still perform the RPF check on
this multicast packet. With the RPF interface as the input interface, the router will create
corresponding entry with the RPF interface as the input interface combining related routing
information, and send this entry to the multicast forwarding table:
If the packet’s actual arriving interface is exactly the RPF interface, the RPF check will pass
and the packet will be forwarded to all the output interfaces;
If the packet’s actual arriving interface is not the RPF interface, the RPF check fails and this
packet will be discarded.
PIM Modes
PIM can be divided into two modes according to different routing mechanisms:
PIM DM: Protocol Independent Multicast-Dense Mode
PIM SM: Protocol Independent Multicast-Sparse Mode
PIM DM
PIM DM is a multicast routing protocol in dense mode. It uses Push Mode to transfer multicast
packets and applies to small network with relatively dense multicast group members.
The working mechanism of PIM DM is illustrated as below:
PIM DM assumes that there is at least one multicast group member in each subnet of the
network, and the multicast packets will be flooded to all the nodes in the network. Then
branches without receivers are pruned from the distribution tree, leaving only branches that
contain receivers. This “flood-and-prune” process takes place periodically. The pruned
branches can also resume to forwarding state periodically.
When a new receiver on a previously pruned branch of the tree joins a multicast group, the PIM
DM takes the Graft (see
Grafting
) mechanism to actively resume this node’s function of
forwarding multicast data, thus reducing the time it takes to resume to the forwarding state.
226