Multicast Routing Algorithms; Multicast Spanning Trees - D-Link DES-3326SR Manual

D-Link DES-3326SR Layer 3 Switch
The router with the lowest IP address is elected as the querier. The explicit group leave message is added to decrease
latency, and routers can ask for membership reports from a particular multicast group ID.
The transition states a host will go through to join or leave a multicast group are shown in the diagram below.

Multicast Routing Algorithms

An algorithm is not a program. An algorithm is a statement of how a problem can be solved. A program is written to
implement an algorithm.
Multicast packets are delivered by constructing multicast trees where the multicast router is the trunk, the branches are the
various subnetworks that may be present, and the leaves are end recipients of the multicast packets. Several algorithms have
been developed to construct these trees and to prune branches that have no active mulitcast group members
Flooding
The simplest algorithm for the delivery of multicast packets is for the multicast router to forward a multicast packet to all
interfaces. This is referred to as flooding. An equally simple refinement of flooding is to have the router check to determine
if a given multicast packet has been received before (in a certain amount of time). If it has, then the packet does not need to
be forwarded at all and can be dropped. If the packet is being received for the first time, it should be flooded to all interface,
except the interface on which it was received. This will ensure that all routers on the network will receive at least one copy
of the multicast packet.
There are some obvious disadvantages to this simple algorithm. Flooding duplicates a lot of packets and uses a lot of
network bandwidth. A multicast router must also keep a record of the multicast packets it has received (for a period of time)
to determine if a given packet has been previously received. So flooding uses a lot of router memory.

Multicast Spanning Trees

A multicast delivery tree that spans the entire network with a single active link between routers (or subnetwork) is called a
multicast spanning tree. Links (or branches) are chosen such that there is only one active path between any two routers.
When a router receives a multicast packet, it forwards the packet on all links except the one on which it was received. This
guarantees that all routers in the network will receive a copy of the packet. The only information the router needs to store is
whether a link is a part of the spanning tree (leads to a router) or not.
Multicast spanning trees do not use group membership information when deciding to forward or drop a given multicast
packet.
204