TP-Link JetStream T2700G-28TQ User Manual page 253

28-port gigabit stackable l2+ managed switch
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RPF Mechanism
PIM uses the unicast routing table to perform the RPF check. RPF mechanism ensures the
multicast packets being forwarded correctly according to the multicast routing configuration, and
avoids loops causing by various reasons.
1. RPF Check
The RPF check relies on unicast route or static multicast route. The unicast routing table
aggregates the shortest paths to each destination network segments, and the static multicast
routing table lists specified static RPF routing entries configured by the user manually. Instead of
maintaining certain unicast routing independently, the multicast routing protocol relies on the
current unicast routing information or static multicast routing in the network to establish multicast
routing entries.
When performing the RPF check, the router will look up the unicast routing table and the static
multicast routing table at the same time. The process is as below:
(1) Chose an optimal route from the unicast routing table and the static routing table respectively:
The router looks up the unicast routing table with the IP address of the packet source as the
destination address, and selects an optimal unicast route automatically. The output interface
of the corresponding entry is the RPF interface, and the next hop is the RPF neighbor. The
router will consider the traveling path of the multicast data sent from the RPF neighbor and
received on the RPF interface as the shortest path from the multicast source S to the local
network.
The router looks up the static multicast routing table with the IP address of the packet source
specified as the source address, and selects an optimal static multicast route automatically.
The corresponding entry explicitly specifies the RPF interface and RPF neighbor.
(2) Select one from the two optimal routes as the RPF route:
According to the longest mask matching principle, the longest mask matching route between them
will be selected; if the two routes have the same mask, the route with higher priority will be
selected; if the two routes also have the same priority, then the static multicast route is prior to the
unicast route.
2. RPF Mechanism Application
When the router receives multicast packets sent from multicast source S to multicast group G, it
will look up the multicast forwarding table at first:
(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;
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