Customer Service Center

• Information from Spirent TestCenter online Help 

IGMP Terminology

• Host Membership Query
  • IGMPv1 multicast routers send Query messages to discover which multicast groups have subscribers located on the attached local networks.
• Host Membership Report
  • An IGMPv1 host sends a Report message (sometimes referred to as a Join message) to join a multicast group or to report a group membership in response to a Query message.
• IGMP
  • Internet Group Management Protocol is used by IPv4 hosts and multicast routers to establish multicast group memberships.
• Leave Group
  • An IGMPv2 host sends a Leave Group message to inform the querier router that it wishes to cancel its membership with a particular multicast group.
• Membership Query
  • IGMPv2 multicast routers send two types of Query messages. A General Query is sent to discover which multicast groups have members on the attached network. A Group-Specific Query is sent to discover if a specific multicast group has any members on the attached network.
  • IGMPv3 adds a third type of Query message. A Group-and-Source-Specific Query is sent to discover if any attached hosts desire (or have) membership with a specific multicast address which has a list of specific sources.
• Querier
  • A multicast router that maintains a list of multicast group memberships for each attached network. There is normally only one querier per physical network. The querier sends out Query messages to determine the multicast group memberships for hosts on the attached network.
• Version 2 Membership Report
  • An IGMPv2 host sends a Report message (sometimes referred to as a Join message) to join a multicast group. IGMPv2 hosts can send a Version 1 Membership Report message for backwards-compatibility.
• Version 3 Membership Report
  • An IGMPv3 host sends a Report message (sometimes referred to as a Join message) to join a multicast group. This Report message may specify that specific sources be used or not used for the multicast group as a condition of membership. IGMPv2 hosts can send a Version 1 or Version 2 Membership Report message for backwards-compatibility.

IGMP Theory

• Internet Group Management Protocol (IGMP) allows IP hosts to report their multicast group membership to adjacent multicast routers. This registration is the first step toward multicast communications.
• When a host wants to participate in a multicast group, it sends an IGMP Report message to its local router. If multiple routers exist on a single segment, they can mutually elect a designated router (querier) that manages all of the IGMP messages for the segment.
• After a router receives one or more membership reports for a specific group, it forwards packets destined for that group to the appropriate interface(s). The router forwards one copy of the data per interface. If there are multiple receivers on a single interface, they all receive the information by monitoring common multicast MAC and IP addresses. Multicast IP addresses are addresses whose first four bits are set to binary 1110 (224.0.0.0 to 239.255.255.255).
• IGMP is a stateful protocol. The router sends periodic queries to the receivers to verify that the hosts want to continue to participate in the multicast groups. These queries are transmitted to a well-known multicast address (224.0.0.1) that is monitored by all systems. If the receivers are still interested in that particular multicast group, they respond with a Membership Report message. When the router stops seeing responses to queries, it deletes the appropriate group from its forwarding table. IGMP version 2 adds a Leave message that gives receivers the ability to gracefully exit from a specific multicast group. This message allows the router to immediately update its forwarding tables, without waiting for the expiration of the query timer.
• IGMPv3 addresses issues associated with multiple sources transmitting to the same group of receivers. IGMPv3 also gives a receiver the ability to specify the source(s) from which it is willing to listen. Sources can be stipulated with include filters in the Report messages, or sources can be specifically rejected with exclude filters. Overall, these filters greatly enhance the security and performance of multicast communications. These filters also add a new dimension to the tables for the participating routers, since they must now keep track of the acceptable sources for every multicast group.
• The specifications for IGMP versions 2 and 3 state that the routers must also support earlier versions of the protocol.

IGMPv3 Include and Exclude mode

• When to use Include or Exclude, "exclude mode is to be used when the list of IPs you want to listen to is longer then the list of IPs you do not want to listen to".
• Since IGMPv1 and v2 do not support source-specific multicast, the same behavior for a "group join" is emulated in IGMPv3 by using exclude mode with an empty exclude list (0 sources).