Qtech QSW-3900-48-SFP-DC [172/313] Multicast packet forwarding
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Qtech QSW-3900-48-SFP-DC [172/313] Multicast packet forwarding

Qtech QSW-3900-24-Т-AC [172/313] Multicast packet forwarding
QTECH Software Configuration Manual
12-171
12.2.4 Multicast Packet Forwarding
12.2.4.1 Multicast Forwarding Tree
Multicast packets travel along tree-shaped forwarding paths known as multicast forwarding trees over the network to
the receivers. Multicast forwarding trees fall into two types: source tree and shared tree.
(1) Source tree
Rooted at the multicast source, a source tree is a forwarding tree with the shortest path from the multicast source to
the receivers; therefore, it is also called a shortest path tree (SPT). An SPT needs to be constructed per source per
group.
As the shortest forwarding path between a multicast source and the receivers, the source tree minimizes the
end-to-end transmission latency. However, this does come at a price. As the router must maintain the routing
information for each multicast source, a great deal of system resource is used and the routing table is very large.
(2) Shared tree
Rooted at a router called a rendezvous point (RP), a shared tree is a forwarding tree with the shortest path from the
RP to each receiver. It is also called a rendezvous point tree (RPT). There is only one RPT per multicast group on the
network. All multicast sources and receivers use the RPT tree for multicast data transmission and reception. The
multicast sources send data to the RP and the RP forwards the data down the RPT to all the receivers.
The main advantage of an RPT is that it allows a router to maintain a small number of routing entries. However, as
the multicast traffic from a multicast source must pass through the RP before it reaches the receivers, this forwarding
tree is not the shortest path from the source to the receivers, and the RP must be highly reliable and powerful.
12.2.4.2 Multicast Packet Forwarding Mechanism
Upon receiving a multicast packet, a router searches its multicast forwarding table according to the
destination address and then forwards the packet accordingly. Forwarding a multicast packet is more complex than
forwarding a unicast packet. In unicast, a router does not care about the source address; it cares only about the
destination address of the packet, based on which the router determines the interface to forward the packet to. In
multicast, multicast traffic is destined for to a group of receivers identified by a logical address known as multicast
address. Upon receiving a multicast packet, a router checks whether the packet has arrived to the correct incoming
interface, namely whether the incoming interface leads to the multicast source, based on the source address before
forwarding the packet out the outgoing interface. This process is known as the reverse path forwarding (RPF) check.
The basis for the RPF check is the existing unicast routing table. The router forwards only those packets
received on the interface connected to the upstream neighbor on the unicast route to the source. This incoming
interface is called RPF interface. The RPF check not only ensures multicast data forwarding along the correct
forwarding path but also helps avoid loops.
The RPF check process is as follows: The router searches the unicast routing table for the RPF interface
toward the multicast source (when an SPT is used) or the RP (when an RPT is used). If the packet is received on the
RPF interface, it passes the RPF check and then forwarded to downstream node; otherwise, the packet is discarded.
12.2.5 Multicast Routing Protocols
Similar to unicast protocols, multicast routing protocols fall into intra-domain and inter-domain protocols:
Based on the group memberships maintained by IGMP, an intra-domain
multicast routing protocol builds multicast distribution trees according to certain

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