4.8 Multicast

Some additional facts about multicasting include:

-Most multicast messages are sent using the UDP protocol. -Frames that contain multicast traffic are sent to a special MAC address. *The MAC address begins with 01-00-5E. The last portion is a form of the IP multicast group address. *A single multicast MAC address can be shared by up to 5 other IP multicast addresses. -A regular switch that receives multicast traffic sends the traffic out all ports because the destination MAC address is an unknown address. *This means that a host might see multicast traffic on its segment, even if it isn't a member of the group. *Hosts that are not members of the group will not process the frame because they will not associate the multicast MAC address with their own address. -IGMP snooping on a switch allows the switch to control the ports that get IGMP traffic for a specific group. *With IGMP snooping, the switch identifies the ports that include members of a specific multicast group. *When a message is received for a group, the message is sent only to the ports that have a group member connected.

IGMP routers keep track of the attached subnets that have group members, using the following process:

1. A router sends a host membership query. This query is addressed to the IP address 224.0.0.1. *The address 224.0.0.1 is never assigned to a group because it is used for the query messages sent by routers.* 2. Hosts that are members of any group respond with a list of the groups they belong to. Each group is identified with a multicast IP address in the range of 224.0.0.0 to 239.255.255.255. *The 224.0.0.0 address range is reserved for local subnets.* 3. The router uses these responses to compile a list of the groups on the subnet that have group members. Routers do not keep track of individual hosts that are members of a group; they only compile a list of groups on the subnet that have at least one member. 4. When a host joins a new group, it automatically sends a join group message to the router. When the last host in a group leaves the group, it sends a leave group message to the router. 5. The IGMP router reports to upstream routers that they have members of a specific group. Upstream routers are the routers that exist between the router and the server that sends out the multicast data stream. They keep track of downstream routers that have group members.

Broadcast

A broadcast message is used to send data to all hosts on the subnet. Broadcast messages: -Are used only on IPv4 networks. -Use the last IP address in each subnet. That IP address is reserved for broadcast messages. -Cannot be forwarded by routers. -Cannot be used to send data to another subnet.

IGMP

A protocol used to identify group members and to forward multicast packets on to the segments where group members reside.

Broadcast

A single packet that, when sent, is processed by all hosts. Broadcast packets are not typically forwarded by routers, so broadcast traffic is limited to within a single subnet.

Unicast

A transmission type that sends messages to a specific host address. The sending device must know the IP address of all recipients and must create a separate packet for each destination device.

Multicast

Multicast is used to send a message to a specific group of devices. Multicasting creates logical groups of hosts. Messages sent to the group are received by all group members. Devices join a multicast group by sending a request to join message to the multicast address. The Internet Group Management Protocol (IGMP) is used to identify group members. It is also used to forward multicast packets to the segments where group members reside.

Anycast

The anycast address is a unicast address that is assigned to more than one interface, typically interfaces that belong to different hosts. An anycast packet is routed to the nearest interface having that address based on routing protocol decisions. Details include the following: -An anycast address is the same as a unicast address. Assigning the same unicast address to more than one interface makes it an anycast address. -The network uses shortest-path calculations to determine the best server to route data to. -Anycast is not technically supported on an IPv4 network. *You can use the Border Gateway Protocol (BGP) to give the same IP address to multiple servers and manually define different routes.* -Anycast is supported on IPv6 networks. -When assigning an anycast address to an interface, you must explicitly identify it as an anycast address to distinguish it from a unicast address.

Multicasting is typically used for streaming video and audio applications, such as video conferencing. It is also used to provide real-time information such as stock market data.

The following process is used when sending a multicast stream: 1. The sending server sends packets addressed to the multicast group. 2. Routers receive the multicast packets and check their lists of group members. -If the router is connected to a subnet that has group members, or if the subnet includes a downstream router with group members, the multicast packet is sent on that subnet. -If a subnet does not have any group members, the packet is not forwarded on that subnet. -If a router does not have any subnets with group members, it drops the packet and does not forward it. 3. Each intermediary router performs the same tasks until the data stream eventually reaches the multicast client.

When communicating over a network, there are different transmission methods you can use. The method you use depends on to whom the data is being sent.

This lesson covers the following topics: Unicast Broadcast Multicast Anycast

Unicast

Unicast transmissions are one-to-one communication between two devices. With a unicast transmission, the sender and receiver must know each other's IP addresses. Data can be easily sent back and forth. A separate packet must be created for each destination device. This is the most common type of transmission used on networks. Unicast works great for communicating between two devices.