What is Internet Group Management Protocol (IGMP) | 7.1 #ENCOR (350-401) MULTICAST PROTOCOLS #CCNP

What is IGMP?

Internet Group Management Protocol (IGMP; Defined in RFC 1112SLN68773_en_US__1iC_External_Link_BD_v1) is a protocol that allows a host to advertise its multicast group membership to neighboring switches and routers. IGMP is a standard protocol used by the TCP/IP protocol suite to achieve dynamic multicasting.

What is Multicasting?

Multicasting is a type of network transmission that allows for communication from one source to a selected group of destinations.

Dynamic multicasting

Proper configuration of dynamic multicasting requires a router or layer-3 aware device that is capable of handling the multicast groups. The layer-3 aware device uses IGMP (Internet Group Management Protocol) to listen for join and leave messages from the clients. The layer-3 device uses join and leave messages to add clients to and remove clients from the multicast group. Many layer-2 devices are capable of IGMP Snooping. IGMP Snooping allows a layer-2 device to “read” the IGMP traffic between hosts and routers (or Queriers), determine when ports join and leave multicast groups, and dynamically forward the traffic to only the ports participating in the group. An IGMP Querier is capable of sending a periodic message (called an IGMP Membership Query) to well-known Multicast IP address 224.0.0.1 (all multicast-capable hosts) at a specified interval. This allows a layer-3 aware device to keep track of which ports belong to which multicast groups.

How IGMP works

When a multicast transmission initiates, the software or service creates a multicast group. This multicast group address consists of an IP address with the first octet in the range 224 – 239 (Class D) and is specified in the IP packet as the destination address for this traffic. The host initiating the transmission sends a message (called an IGMP Membership Report) to the 224.0.0.2 address (all multicast routers) specifying the multicast group address. The switch receives this message, adds the multicast group to its table, and adds the receiving port as a member of the group. It also forwards this report to any multicast routers. The router then adds these hosts to the multicast routing table. All hosts wishing to be members of the group also send join messages. The switch intercepts these messages and adds the receiving ports as members of the group. It forwards these messages to the multicast router, as well. All traffic sent to the multicast destination address is forwarded only to the ports participating in the specified group. In an effort to keep the membership information current, the IGMP querier continues to send Membership Queries. All hosts that wish to remain in the group must reply to these queries. If the hosts in the group do not reply within a specified time period, the switch removes those ports from the group table. Once all members have left the multicast group, the switch removes the multicast address from its table.