IP Multicast


Many types of data can be transferred between devices over an IP network, including, for example, document files, voice, and video. However, a traditional IP network is not efficient when sending the same data to many locations; the data is sent in unicast packets and is therefore replicated on the network for each destination. For example, if a CEO's annual video address is sent out on a company's network for all employees to watch, the same data stream must be replicated for each employee. Obviously, this would consume many resources, including precious WAN bandwidth.

IP multicast technology enables networks to send data to a group of destinations in the most efficient way. The data is sent from the source as one stream; this single data stream travels as far as it can in the network. Devices only replicate the data if they need to send it out on multiple interfaces to reach all members of the destination group.

Multicast groups are identified by Class D IP addresses, which are in the range from 224.0.0.0 to 239.255.255.255. IP multicast involves some new protocols for network devices, including two for informing network devices which hosts require which multicast data stream and one for determining the best way to route multicast traffic. These three protocols are described in the following sections.

Internet Group Management Protocol (IGMP) and Cisco Group Management Protocol (CGMP)

IGMP is used between hosts and their local routers. Hosts register with the router to join (and leave) specific multicast groups; the router is then aware that it needs to forward the data stream destined to a specific multicast group to the registered hosts.

In a typical network, hosts are not directly connected to routers but are connected to a Layer 2 switch, which is in turn connected to the router. IGMP is a network layer, Layer 3, protocol. Thus, Layer 2 switches do not participate in IGMP and are therefore not aware of which hosts attached to them might be part of a particular multicast group. By default, Layer 2 switches flood multicast frames to all ports (except the port from which the frame originated), which means that all multicast traffic received by a switch would be sent out on all ports, even if only one device on one port required the data stream. Cisco therefore developed CGMP, which is used between switches and routers. The routers inform each of their directly connected switches of IGMP registrations that were received from hosts through the switch, in other words, from hosts accessible through the switch. The switch then forwards the multicast traffic only to ports that those requesting hosts are on, rather than flooding the data to all ports. (Switches, including non-Cisco switches, can alternatively use IGMP snooping to eavesdrop on the IGMP messages sent between routers and hosts to learn similar information.)

Figure 10-1 illustrates the interaction of these two protocols. Hosts A and D register, using IGMP, to join the multicast group to receive data from the server. The router informs both switches of these registrations, using CGMP. When the router forwards the multicast data to the hosts, the switches ensure that the data only goes out of the ports on which hosts A and D are connected. The ports on which hosts B and C are connected do not receive the multicast data.

Figure 10-1. GMP and CGMP Inform Network Devices About Which Hosts Want Which Multicast Data


Protocol Independent Multicast (PIM) Routing Protocol

PIM is used by routers that are forwarding multicast packets. The "protocol independent" part of the name indicates that PIM is independent of the unicast routing protocol (for example, Enhanced Interior Gateway Routing Protocol [EIGRP] or Open Shortest Path First [OSPF]) running in the network. PIM uses the normal routing table, populated by the unicast routing protocol, in its multicast routing calculations.

Note

EIGRP, OSPF, and so forth are called unicast routing protocols because they are used for creating and maintaining unicast routing information in the routing table. Recall, though, that they use multicast packets (or broadcast packets in some protocols) to send their routing update traffic.

(Note that a variant of OSPF, called multicast OSPF, supports multicast routing. Cisco routers do not support multicast OSPF.)


Note

Unlike other routing protocols, no routing updates are sent between PIM routers.


When a router is forwarding a unicast packet, it looks up the destination address in its routing table and forwards the packet out of the appropriate interface. However, when forwarding a multicast packet, the router might have to forward the packet out of multiple interfaces, toward all the receiving hosts. Multicast-enabled routers use PIM to dynamically create distribution trees that control the path that IP multicast traffic takes through the network to deliver traffic to all receivers. The following two types of distribution trees exist:

  • Source tree A source tree is created for each source sending to each multicast group. The source tree has its root at the source and has branches through the network to the receivers.

  • Shared tree A shared tree is a single tree that is shared between all sources for each multicast group. The shared tree has a single common root, called a rendezvous point (RP).

Multicast routers consider the source address of the multicast packet as well as the destination address, and use the distribution tree to forward the packet away from the source toward the destination. Forwarding multicast traffic away from the source, rather than to the receiver, is called Reverse Path Forwarding (RPF). To avoid routing loops, RPF uses the unicast routing table to determine the upstream (toward the source) and downstream (away from the source) neighbors and ensures that only one interface on the router is considered to be an incoming interface for data from a specific source. (For example, data received on one router interface and forwarded out another interface can loop around the network and come back into the same router on a different interface; RPF ensures that this data is not forwarded again.)

PIM operates in one of the following two modes:

  • Sparse mode This mode uses a "pull" model to send multicast traffic. Sparse mode uses a shared tree and therefore requires an RP to be defined. Sources register with the RP. Routers along the path from active receivers that have explicitly requested to join a specific multicast group register to join that group. These routers calculate, using the unicast routing table, whether they have a better metric to the RP or to the source itself; they forward the join message to the device with which they have the better metric.

  • Dense mode This mode uses a "push" model that floods multicast traffic to the entire network. Dense mode uses source trees. Routers that have no need for the data (because they are not connected to receivers that want the data or to other routers that want it) request that the tree is pruned so that they no longer receive the data.

Note

Further information on IP multicast can be found at http://www.cisco.com/go/ipmulticast.





Campus Network Design Fundamentals
Campus Network Design Fundamentals
ISBN: 1587052229
EAN: 2147483647
Year: 2005
Pages: 156

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