21.0 Multicasting...more!
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
appropriately.
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 124.76.12.3 is
Router(config)#ip pim rp-address 124.76.12.3
To remove an RP address, use the command
no ip pim rp-address
124.76.12.3
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.
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