IP multicast transmits messages to multiple recipients at the same time. This one-to-many delivery mechanism is similar to broadcasting, except that multicasting transmits to specific groups and broadcasting transmits to all receivers on a network. Because IP multicast transmits only one stream of data to the network where it replicates to many receivers, multicasting saves a considerable amount of bandwidth.
IP multicast services benefit applications such as video conferencing, dissemination of datagram information, and dissemination of mail or news to a large number of recipients.
Multicast protocols use different techniques to discover delivery paths.
A distribution tree is a set of multicast routers and subnetworks that permit the members of a group to receive traffic from a source. The source of the tree depends on the algorithm used by the multicast protocol. The following diagram is an example of a simple distribution tree where S is the multicast source and the arrows indicate the multicast broadcast procedure.
Broadcast and prune methods use multicast traffic to build the distribution tree. Periodically, the source sends or broadcasts data to the extremities of the internetwork to search for active group members. If no local members of the group exist, the router sends a message to the host, removing itself from the distribution tree, and thus pruning the router.
The following diagram illustrates how the host prunes routers from the distribution tree. First, the router sends a message to the source, after which the pruned routers do not receive multicast data.
Reverse path multicast is based on the concept that a multicast distribution tree is built on the shortest path from the source to each subnetwork that contains active receivers. After a datagram arrives on an interface, the router determines the reverse path to the source of the datagram by examining the routing table of known network sources. If the datagram is not on the optimal delivery tree, the router discards it.
Multicast host groups and their group members enable the IP multicast router to transmit just to those groups interested in receiving the traffic. The switch uses the Internet Group Membership Protocol (IGMP) to learn the existence of host group members on their directly attached subnets. A router communicates with the hosts on a local network by sending IGMP queries. Hosts respond by issuing IGMP reports. For more information about host groups, see Multicast host groups and Multicast addresses. For more information about IGMP, see Internet Group Management Protocol.
Multicast traffic forwarding transmits frames to all interfaces or subnets for which it receives IGMP reports for the multicast group indicated in the destination IP address. Multicast packets forwarded within the same virtual LAN (VLAN) remain unchanged. The switch does not forward packets to networks that do not use members of the multicast group indicated in the destination IP address.
IP multicast is a method for addressing, routing, and delivering a datagram to a collection of receivers called a host group.
Host groups are permanent or transient, with the following characteristics:
A permanent host group uses a well-known, administratively assigned IP multicast group address. This address is permanent and defines the group. A permanent host group can consist of zero or more members.
A transient host group exists only as long as members need its services. IP addresses in the multicast range that are not reserved for permanent groups are available for dynamic assignment to transient host groups.
A host system on an IP network sends a message to a multicast group by using the IP multicast address for the group. To receive a message addressed to a multicast group, however, the host must be a member of the group and must reside on a network where that group is registered with a local multicast router.
An IP multicast host group can consist of zero or more members and places no restrictions on its membership. Host members can reside anywhere, they can join and leave the group at any time, and they can be members of more than one group at the same time.
In general, hosts that are members of the same group reside on different networks. However, a range of multicast addresses (224.0.0.x) is reserved for locally-scoped groups. All message traffic for these hosts typically remains on the local network. Hosts that belong to a group in this address range and that reside in different networks do not receive message traffic for each other.
Important
You can apply a special set of filters (global filters) to multicast packets. You can also create, deny, or accept filters to configure the sources that can receive and send data. For more information about how to configure filters, see Traffic filtering fundamentals.
Each host group uses a unique multicast address. To reach all members of the group, a sender uses the multicast address as the destination address of the datagram.
An IP version 4 multicast address is a Class D address (the high-order bits are 1110) from 224.0.0.0 to 239.255.255.255. These addresses are assigned statically for use by permanent groups and dynamically for use by transient groups.
The block of addresses from 224.0.0.1 to 224.0.0.255 is reserved for routing protocols and other low-level protocols. Multicast routers do not forward datagrams with addresses in this range because the time-to-live (TTL) value for the packet is usually 1.
You can use the following protocols to enable multicast routing on a switch:
Internet Group Management Protocol (IGMP)—learns the existence of host group members on directly attached subnets.
Multicast Router Discovery (MRDISC) protocol—discovers multicast routers in a Layer 2 bridged domain configured for IGMP snoop.
Protocol Independent Multicast (PIM)
Sparse Mode (PIM-SM) protocol—suitable for implementation on networks sparsely populated by receivers.
Source Specific Multicast (PIM-SSM) protocol—uses a one-to-many model where members can receive traffic from one or more specific sources. This protocol is suitable for television channels and other content-distribution applications.
Use static source groups to configure static source-group entries in the PIM-SM, or PIM-SSM multicast routing table. PIM cannot prune these entries from the distribution tree. In other words, even if no receivers for the group exist, the multicast stream for a static source-group entry stays active. PIM never prunes static forwarding entries. If you no longer need the entries, you must manually delete them.
To configure static source groups, you must first globally enable PIM. If you disable PIM, the switch saves all of the configured static source-group entries and deactivates them. After you re-enable PIM, the switch reactivates the static source groups.
Static source groups ensure that the multicast route (mroute) records remain in the distribution tree. After receivers join the group, they do not experience a delay in receiving multicast data because they do not need to graft onto the group, or start a join process in the case of PIM. This timing is essential for applications where the multicast data must send to a receiver as soon as the receiver joins the group, for example, when a switch delivers television channels to receivers. After the receiver turns the channel, which is equivalent to joining a group, the receiver can view the channel immediately.
Static entries result in continuous traffic if the source is active, even if no receivers exist. However, the system does not forward traffic with a static entry if no receivers exist, but forwards it continuously to the switch where the entry is programmed and crosses intermediate switches on the path.
You can configure static source-group entries for a specific source or subnet. If several sources on the same subnet send traffic to the same group, traffic for all these sources flows continuously when using the subnet configuration.
After you configure static source groups, keep the following points in mind:
If you disable PIM, the switch deactivates all of the static source groups. After you re-enable PIM, the switch activates the static source groups.
In PIM-SM configuration, the static source-group feature works for both specific source addresses and subnet addresses by using the SrcSubnetMask field.
When the network mask is 255.255.255.255, the full source address is used to match the (S,G) which is the specific source case. When the network mask field is a subnet mask for the source, only the source subnet is used to match (S,G)s.
In PIM-SSM configurations, static source groups have the following limitations:
Subnets: SSM static source groups work only with specific IP addresses. Static source groups cannot work with source subnets, so the mask must use a full 32-bit mask, 255.255.255.255, and the source must use a host address.