Most of us are very familiar with p2p networks concept. They help to share or download files from many different sources at the same time, some of them are determining the closest source location before the download process starts. Even though the IP multicast works a little bit different than that, the concept and the goal is the same: to optimize the traffic and to utilize network connection more efficient if possible.
IP communication allows a host to send packets in two manner:
1. To a single host (called unicast transmission)
2. To all hosts (called broadcast transmission)
IP multicast provides a third possibility:
3. To a subset of all hosts (called a group transmission)
IP multicast is a bandwidth conserving technology that redueces traffic by simultaneously delivering a single stream of information to potentially thousands of corporate recipients and homes. Applications that take advantage of multicast include video conferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news.
IP multicast delivers application source traffic to multiple receivers without burdening the source or the receivers while using a minimum of network bandwidth. Multicast packets are replicated in the network at the point where paths diverge by Cisco routers enabled with Protocol Independent Multicast (PIM) and other supporting multicast protocols, resulting in the most efficient delivery of data to multiple receivers.
Many alternatives to IP multicast require the source to send more than one copy of the data. Some, such as application-level multicast, require the source to send an individual copy to each receiver. Even low-bandwidth applications can benefit from using Cisco IP multicast when there are thousands of receivers. High-bandwidth applications, such as MPEG video, may require a large portion of the available network bandwidth for a single stream. In these applications, IP multicast is the only way to send to more than one receiver simultaneously.
IP multicast addresses specify a “set” of IP hosts that have joined a group and are interested in receiving multicast traffic designated for that particular group. The Internet Assigned Numbers Authority (IANA) controls the assignment of IP multicast addresses. IANA has assigned the IPv4 Class D address space to be used for IP multicast. Therefore, all IP multicast group addresses fall in the range from 224.0.0.0 through 239.255.255.255.
The most important terms are:
RP (Rendezvous Point) - it is designated router in your network that is usually the "center" of it. It receives and decides which path to choose to deliver packet to all receivers that are interested.
PIM (Protocol Independent Multicast) - is IP routing protocol-independent and can leverage whichever unicast routing protocols are used to populate the unicast routing table, including Enhanced Interior Gateway Routing Protocol (EIGRP), Open Shortest Path First (OSPF), Border Gateway Protocol (BGP), and static routes. PIM uses this unicast routing information to perform the multicast forwarding function. Although PIM is called a multicast routing protocol, it actually uses the unicast routing table to perform the RPF check function instead of building up a completely independent multicast routing table. Unlike other routing protocols, PIM does not send and receive routing updates between routers.
PIM-DM (PIM Dense Mode) - uses a push model to flood multicast traffic to every corner of the network. This push model is a brute force method for delivering data to the receivers. This method would be efficient in certain deployments in which there are active receivers on every subnet in the network.
PIM-SM (PIM Sparse Mode) - uses a pull model to deliver multicast traffic. Only network segments with active receivers that
have explicitly requested the data will receive the traffic.
Bidir-PIM (Bidirectional PIM) - is an enhancement of the PIM protocol that was designed for efficient many-to-many communications within an individual PIM domain. Multicast groups in bidirectional mode can scale to an arbitrary number of sources with only a minimal amount of additional overhead.
More information visit a Cisco documentation page, for a configuration guides click here.
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