Chapter 16: Introduction to IP Multicasting

IGMPv2 Host Membership Query

A message sent by a multicast router, by default every 125 seconds, on each of its LAN interfaces to determine whether any host wants to receive multicast traffic for any group.

IGMPv2 Group-Specific Query

A message sent by a router, after receiving a Leave message from a host, to determine whether there are still any active members of the group. The router uses the group address as the destination address.

IGMPv2 Host Membership Report

A message sent by each host, either in response to a router Query or on its own, to all multicast groups for which it would like to receive multicast traffic.

IGMP snooping

A method for optimizing the flow of multicast IP packets passing through a LAN switch. The switch using IGMP snooping examines IGMP messages to determine which ports need to receive traffic for each multicast group.

multicast MAC address

A 48-bit address that is calculated from a Layer 3 multicast address by using 0x0100.5E as the multicast vendor code (OUI) for the first 24 bits, always binary 0 for the 25th bit, and copying the last 23 bits of the Layer 3 multicast address.

IGMPv2 Leave

A message sent by a host when it wants to leave a group, addressed to the All Multicast Routers address 224.0.0.2.

CGMP

Cisco Group Management Protocol. A Cisco-proprietary feature. After a Cisco multicast router receives IGMP Join or Leave messages from hosts, it communicates to the connected Cisco switches, telling them which hosts (based on their unicast MAC addresses) have joined or left each multicast group. Switches examine their CAM tables and determine on which ports these hosts are connected and either forward multicast traffic or stop forwarding on those ports only.

multicast address range

IP multicast address range from 224.0.0.0 through 239.255.255.255.

IGMP

Internet Group Management Protocol. A communication protocol between hosts and a multicast router by which routers learn of which multicast groups' packets need to be forwarded onto a LAN.

MRT

Maximum Response Time. After a host receives an IGMP Query, the amount of time (default, 10 seconds) the host has to send the IGMP Report.

transient multicast group

Multicast addresses that are not assigned by IANA.

RGMP

Router-Port Group Management Protocol A Cisco-proprietary Layer 2 protocol that enables a router to communicate to a switch which multicast group traffic the router does and does not want to receive from the switch.

multicasting

Sending a message from a single source or multiple sources to selected multiple destinations across a Layer 3 network in one data stream.

SSM

Source-Specific Multicast (SSM). Receivers subscribe to an (S,G) channel when they request to join a multicast group. That is, they specify the unicast IP address of their multicast source and the group multicast address. SSM is typically used in very large multicast deployments such as television video.

multicast address structure

The first 4 bits of the first octet must be 1110. The last 28 bits are unstructured.

permanent multicast group

The multicast addresses assigned by IANA.

joining a group

The process of installing a multicast application; also referred to as launching an application.

source-specific addresses

The range 232.0.0.0 through 232.255.255.255 that is allocated by IANA for SSM destination addresses and is reserved for use by source-specific applications and protocols.

GLOP addressing

The range 233.0.0.0 through 233.255.255.255 that IANA has reserved (RFC 2770) on an experimental basis. It can be used by anyone who owns a registered autonomous system number to create 256 global multicast addresses.

administratively scoped addresses

The range 239.0.0.0 through 239.255.255.255 that IANA has assigned for use in private multicast domains.

Report Suppression mechanism

When a Query is received from a router, each host randomly picks a time between 0 and the Maximum Response Time period to send a Report. When the host with the smallest time period first sends the Report, the rest of the hosts suppress their reports.

querier election

When multiple routers are connected to a subnet, only one should be sending IGMP queries. It is called a querier. IGMPv1 does not have any rules for electing a querier. In IGMPv2 and IGMPv3, a router with the lowest interface IP address on the subnet is elected as a querier.