In IP Multicasting, one copy of each packet is sent to host group.
The packets are replicated on as needed basis. On a single subnet,
a packet is sent only once. All hosts that belong to the host group
receive the Multicast traffic. This is in contrast to Unicast, where
the packets are replicated for each host. For example, in Multicast
network, assuming that there are 3 hosts in a host-group that belong
to the same subnet, and a streaming video bandwidth of 300Kbps,
the effective Multicast bandwidth will be 300Kbps. If the same video
is streamed using Unicast, the bandwidth usage is linearly proportional
to the number of hosts. Therefore, the bandwidth required will be
3X300Kbps ( Video is streamed once for each host!).
In the case of broadcast, only one copy of a packet is sent to all hosts on the network. Compare this with replicating the packets on as needed basis in Multicasting. Multicast packets are not propagated on subnets that do not want to receive the Multicast.
Internet's Multicast backbone (known as MBONE) uses DVMRP (Distance
Vector Multicast Routing protocol) for Multicasting.
PIM is a Multicast routing protocol (just as DVMRP and MOSPF) supported by Cisco routers, and used between routers. Cisco routers does not support DVMRP and MOSPF. However, Cisco routers support PIM to DVMRP interaction, so that DVMRP packets are read, and necessary action is taken.
CGMP is a protocol used between Cisco routers and Cisco Catalyst switches.
Cisco routers do not support MOSPF.
IGMP is a Multicast protocol, that is used between the host and a router.
22.0 Requirements for enabling IP Multicasting:
The following are broad requirements for enabling IP Multicasting
on a network:
1. The applications running on the servers and receiving computers must support Multicasting.
2. Network Interface Cards (NICs) on receiving computers must be capable of monitoring and receiving Multicast traffic. Usually, NICs monitor only for Unicast and Broadcast traffic. Ensure that the NICs support Multicast traffic.
3. It is desirable that the network participating in IP Multicasting has sufficient resources, in the form of computing power and bandwidth.
4. The routers and switches participating
in the Multicast must be capable of directing Multicast traffic
5. Also, remember that the IP protocol stack supports Multicasting.
Without CGMP, Multicast traffic is transmitted to all the ports within the VLAN on which the Multicast frames are received. Cisco Group Management Protocol (CGMP) enables Catalyst switches to make forwarding decisions based on the Internet Group Management Protocol (IGMP) information provided by the Cisco routers. When CGMP is enabled, IP Multicast traffic is transmitted only to Catalyst switch ports that have hosts registered for receiving Multicast traffic. This enables efficient resource utilization in the network.
23.0 Support for Multicasting by Cisco Routers:
The following may be noted regarding support for Multicasting
by Cisco routers:
1. Cisco routers do not support MOSPF
2. Cisco routers can not run DVMRP, but can interoperate with DVMRP.
3. Cisco routers run PIM (Protocol Independent Multicasting).
4. If PIM Dense mode is selected, the multicast-enabled router floods the multicast packets to all interfaces in the Outgoing Interface LIST (OILIST).
5. If PIM Sparse mode is selected, the Multicast interfaces are added to the OILIST only after the join messages are received from downstream routers/ or when there are directly connected receivers.
24.0 Some of the important command to know in IP Multicasting:
1. Router#show ip mroute: This command displays the Multicast routing information. A variation of this command
2. Router#show ip mroute <host ip> <group ip>
The above command displays the contents of the Multicast routing table for a specified IP host address and specified IP address.
3. Show cgmp: This command shows the current CGMP entries such as VLAN number, Addresses, and Destinations.
4. Show cgmp statistics <vlan>: This command displays the CGMP statistics on a per-VLAN basis. The messages include valid cgmp joins received, cgmp leaves received, queries received etc.
5. Show ip igmp interface vlan <n>, where n is the desired vlan number.
The above command displays the current interface status. This include whether CGMP is enabled on the interface, IGMP is enabled on the interface, IGMP query interval etc.
6. The command used to assign the RP (Rendezvous Point) address
of 126.96.36.199 is
Router(config)#ip pim rp-address 188.8.131.52
To remove an RP address, use the command
no ip pim rp-address 184.108.40.206
25.0 An interface is placed in the OILIST if one of the
following conditions are met:
1. A PIM neighbor is heard on an interface.
2. A host connected to the interface has joined a Multicast group.
3. The interface is manually configured to join a Multicast group.
Note that, enabling Multicasting globally on a router alone does not enable an interface to be placed in OILIST.