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IP multicast is a bandwidth-conserving technology that reduces traffic by delivering a single stream of information simultaneously to potentially thousands of businesses 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 routing enables a host (source) to send packets to a group of hosts (receivers) anywhere within the IP network by using a special form of IP address called the IP multicast group address. The sending host inserts the multicast group address into the IP destination address field of the packet and IP multicast routers and multilayer switches forward incoming IP multicast packets out all interfaces that lead to the members of the multicast group. Any host, regardless of whether it is a member of a group, can send to a group. However, only the members of a group receive the message.
IP Multicast Routing
The software supports
the following protocols to implement IP multicast routing:
IGMP is used
between hosts on a LAN and the routers
devices) on that LAN to track the
multicast groups of which hosts are members.
To participate in IP
multicasting, multicast hosts, routers, and multilayer
devices must have the Internet Group
Management Protocol (IGMP) operating.
Independent Multicast (PIM) is used between routers so that they can track
which multicast packets to forward to each other and to their directly
IGMP Snooping is
used for multicasting in a Layer 2 switching environment. It helps reduce the
flooding of multicast traffic by dynamically configuring Layer 2 interfaces so
that multicast traffic is forwarded to only those interfaces associated with IP
This figure shows
where these protocols operate within the IP multicast environment.
Figure 1. IP Multicast Routing Protocols
According to IPv4 multicast
standards, the MAC destination multicast address begins with 0100:5e and is
appended by the last 23 bits of the IP address. For example, if the IP
destination address is 188.8.131.52, the MAC destination address is
A multicast packet is
unmatched when the destination IPv4 address does not match the destination MAC
device forwards the unmatched packet in
hardware based upon the MAC address table. If the destination MAC address is
not in the MAC address table, the
device floods the packet to the all port in
the same VLAN as the receiving port.
Multicast Group Transmission Scheme
IP communication consists of hosts that act as senders and receivers of traffic as shown in the first figure. Senders are called sources. Traditional IP communication is accomplished by a single host source sending packets to another single host (unicast transmission) or to all hosts (broadcast transmission). IP multicast provides a third scheme, allowing a host to send packets to a subset of all hosts (multicast transmission). This subset of receiving hosts is called a multicast group. The hosts that belong to a multicast group are called group members.
Multicast is based on this group concept. A multicast group is an arbitrary number of receivers that join a group in order to receive a particular data stream. This multicast group has no physical or geographical boundaries--the hosts can be located anywhere on the Internet or on any private internetwork. Hosts that are interested in receiving data from a source to a particular group must join that group. Joining a group is accomplished by a host receiver by way of the Internet Group Management Protocol (IGMP).
In a multicast environment, any host, regardless of whether it is a member of a group, can send to a group. However, only the members of a group can receive packets sent to that group. Multicast packets are delivered to a group using best-effort reliability, just like IP unicast packets.
In the next figure, the receivers (the designated multicast group) are interested in receiving the video data stream from the source. The receivers indicate their interest by sending an IGMP host report to the routers in the network. The routers are then responsible for delivering the data from the source to the receivers. The routers use Protocol Independent Multicast (PIM) to dynamically create a multicast distribution tree. The video data stream will then be delivered only to the network segments that are in the path between the source and the receivers.
IP Multicast Boundary
As shown in the
figure, address scoping defines domain boundaries so that domains with RPs that
have the same IP address do not leak into each other. Scoping is performed on
the subnet boundaries within large domains and on the boundaries between the
domain and the Internet.
Figure 2. Address Scoping at
You can set up an
administratively scoped boundary on an interface for multicast group addresses
ipmulticastboundary command with the
argument. A standard access list defines the range of addresses affected. When
a boundary is set up, no multicast data packets are allowed to flow across the
boundary from either direction. The boundary allows the same multicast group
address to be reused in different administrative domains.
The Internet Assigned
Numbers Authority (IANA) has designated the multicast address range 184.108.40.206
to 220.127.116.11 as the administratively scoped addresses. This range of
addresses can be reused in domains administered by different organizations.
They would be considered local, not globally unique.
You can configure the
keyword to examine and filter Auto-RP discovery and announcement messages at
the administratively scoped boundary. Any Auto-RP group range announcements
from the Auto-RP packets that are denied by the boundary access control list
(ACL) are removed. An Auto-RP group range announcement is permitted and passed
by the boundary only if all addresses in the Auto-RP group range are permitted
by the boundary ACL. If any address is not permitted, the entire group range is
filtered and removed from the Auto-RP message before the Auto-RP message is
IP Multicast Group Addressing
A multicast group is identified by its multicast group address. Multicast packets are delivered to that multicast group address. Unlike unicast addresses that uniquely identify a single host, multicast IP addresses do not identify a particular host. To receive the data sent to a multicast address, a host must join the group that address identifies. The data is sent to the multicast address and received by all the hosts that have joined the group indicating that they wish to receive traffic sent to that group. The multicast group address is assigned to a group at the source. Network administrators who assign multicast group addresses must make sure the addresses conform to the multicast address range assignments reserved by the Internet Assigned Numbers Authority (IANA).
IP multicast addresses have been assigned to the IPv4 Class D address space by IANA. The high-order four bits of a Class D address are 1110. Therefore, host group addresses can be in the range 18.104.22.168 to 22.214.171.124. A multicast address is chosen at the source (sender) for the receivers in a multicast group.
The Class D address range is used only for the group address or destination address of IP multicast traffic. The source address for multicast datagrams is always the unicast source address.
IP Multicast Address Scoping
The multicast address range is subdivided to provide predictable behavior for various address ranges and for address reuse within smaller domains. The table provides a summary of the multicast address ranges. A brief summary description of each range follows.
Table 1 Multicast Address Range Assignments
Reserved Link-Local Addresses
126.96.36.199 to 188.8.131.52
Reserved for use by network protocols on a local network segment.
Globally Scoped Addresses
184.108.40.206 to 220.127.116.11
Reserved to send multicast data between organizations and across the Internet.
Source Specific Multicast
18.104.22.168 to 22.214.171.124
Reserved for use with the SSM datagram delivery model where data is forwarded only to receivers that have explicitly joined the group.
126.96.36.199 to 188.8.131.52
Reserved for statically defined addresses by organizations that already have an assigned autonomous system (AS) domain number.
Limited Scope Address
184.108.40.206 to 220.127.116.11
Reserved as administratively or limited scope addresses for use in private multicast domains.
Reserved Link-Local Addresses
The IANA has reserved the range 18.104.22.168 to 22.214.171.124 for use by network protocols on a local network segment. Packets with an address in this range are local in scope and are not forwarded by IP routers. Packets with link local destination addresses are typically sent with a time-to-live (TTL) value of 1 and are not forwarded by a router.
Within this range, reserved link-local addresses provide network protocol functions for which they are reserved. Network protocols use these addresses for automatic router discovery and to communicate important routing information. For example, Open Shortest Path First (OSPF) uses the IP addresses 126.96.36.199 and 188.8.131.52 to exchange link-state information.
IANA assigns single multicast address requests for network protocols or network applications out of the 224.0.1.xxx address range. Multicast routers forward these multicast addresses.
Globally Scoped Addresses
Addresses in the range 184.108.40.206 to 220.127.116.11 are called globally scoped addresses. These addresses are used to send multicast data between organizations across the Internet. Some of these addresses have been reserved by IANA for use by multicast applications. For example, the IP address 18.104.22.168 is reserved for Network Time Protocol (NTP).
Source Specific Multicast Addresses
Addresses in the range 22.214.171.124/8 are reserved for Source Specific Multicast (SSM) by IANA. In Cisco IOS software, you can use the
ippimssmcommand to configure SSM for arbitrary IP multicast addresses also. SSM is an extension of Protocol Independent Multicast (PIM) that allows for an efficient data delivery mechanism in one-to-many communications. SSM is described in the
IP Multicast Delivery Modes section.
GLOP addressing (as proposed by RFC 2770, GLOP Addressing in 233/8) proposes that the 126.96.36.199/8 range be reserved for statically defined addresses by organizations that already have an AS number reserved. This practice is called GLOP addressing. The AS number of the domain is embedded into the second and third octets of the 188.8.131.52/8 address range. For example, AS 62010 is written in hexadecimal format as F23A. Separating the two octets F2 and 3A results in 242 and 58 in decimal format. These values result in a subnet of 184.108.40.206/24 that would be globally reserved for AS 62010 to use.
Limited Scope Addresses
The range 220.127.116.11 to 18.104.22.168 is reserved as administratively or limited scoped addresses for use in private multicast domains. These addresses are constrained to a local group or organization. Companies, universities, and other organizations can use limited scope addresses to have local multicast applications that will not be forwarded outside their domain. Routers typically are configured with filters to prevent multicast traffic in this address range from flowing outside an autonomous system (AS) or any user-defined domain. Within an AS or domain, the limited scope address range can be further subdivided so that local multicast boundaries can be defined.
Network administrators may use multicast addresses in this range, inside a domain, without conflicting with others elsewhere in the Internet.
Layer 2 Multicast Addresses
Historically, network interface cards (NICs) on a LAN segment could receive only packets destined for their burned-in MAC address or the broadcast MAC address. In IP multicast, several hosts need to be able to receive a single data stream with a common destination MAC address. Some means had to be devised so that multiple hosts could receive the same packet and still be able to differentiate between several multicast groups. One method to accomplish this is to map IP multicast Class D addresses directly to a MAC address. Using this method, NICs can receive packets destined to many different MAC address.
Cisco Group Management Protocol (
CGMP) is used on routers connected to Catalyst switches to perform tasks similar to those performed by IGMP. CGMP is necessary for those Catalyst switches that cannot distinguish between IP multicast data packets and IGMP report messages, both of which are addressed to the same group address at the MAC level.
IP Multicast Delivery Modes
IP multicast delivery modes differ only for the receiver hosts, not for the source hosts. A source host sends IP multicast packets with its own IP address as the IP source address of the packet and a group address as the IP destination address of the packet.
Source Specific Multicast (SSM) is a datagram delivery model that best supports one-to-many applications, also known as broadcast applications. SSM is a core network technology for the Cisco implementation of IP multicast targeted for audio and video broadcast application environments.
For the SSM delivery mode, an IP multicast receiver host must use IGMP Version 3 (IGMPv3) to subscribe to channel (S,G). By subscribing to this channel, the receiver host is indicating that it wants to receive IP multicast traffic sent by source host S to group G. The network will deliver IP multicast packets from source host S to group G to all hosts in the network that have subscribed to the channel (S, G).
SSM does not require group address allocation within the network, only within each source host. Different applications running on the same source host must use different SSM groups. Different applications running on different source hosts can arbitrarily reuse SSM group addresses without causing any excess traffic on the network.