A multicast routing protocol is responsible for
the construction of multicast delivery trees and enabling multicast
packet forwarding. Different IP multicast routing protocols use
different techniques to construct multicast Spanning Trees and
forward packets.
IP multicast routing protocols generally
follow one of two basic approaches, depending on the distribution of
multicast group members on the network.
The first approach is based on the assumption
that the multicast group members are densely distributed throughout
the network and bandwidth is plentiful, meaning that almost all
hosts on the network belong to the group. These dense-mode multicast
routing protocols rely on periodic flooding of the network with
multicast traffic to set up and maintain the distribution tree.
Dense-mode routing protocols include the
following:
- Distance Vector Multicast Routing Protocol
(DVMRP)
- Multicast Open Shortest Path First (MOSPF)
- Protocol Independent Multicast - Dense Mode
(PIM-DM)
Note: DVMRP was the first true multicast routing protocol; it is a classless distance vector based (sending updates every 60 seconds) multicast routing protocol with a maximum hop count of 32. It has been largely replaced by protocols such as PIM, just as RIP has been replaced with protocols such as OSPF.
PIM is a routing protocol-independent multicasting protocol that does not use periodic updates, is classless, and does not require a separate multicast routing protocol. It has two flavors -
sparse and dense - sparse mode tends to be used in a WAN environment and dense mode tends to be used in a LAN environment.
Dense-mode protocols
employ only SPTs to deliver (S, G) multicast traffic using a push
principle. The push principle assumes that every subnet in the
network has at least one receiver of the (S, G) multicast traffic,
and therefore, the traffic is pushed or flooded to all points in the
network. This process is analogous to a radio or TV broadcast that
is transmitted over the air to all homes within the coverage area.
Receivers simply need to tune in to the broadcast to receive the
program.
The DM protocols are most appropriate in
environments with densely clustered receivers and the bandwidth to
tolerate flooding. An example of when a DM protocol may be used is
when a CEO of a company wants to broadcast a message to all
employees within the headquarters campus network.