What Is IGMP


In this article, we will deeply examine the wonderful protocol : IGMP ! First of all we need to find that answer who the hell is this guy, what is IGMP ? Let’s take a look at.

IGMP is used to dynamically register individual hosts in a multicast group on a particular LAN.

IGMP Hosts

Hosts identify group memberships by sending IGMP messages to their local multicast router.

IGMP Routers

Routers listen to IGMP messages and periodically send out queries to discover which groups are active or inactive on a particular subnet.

igmp topology

Like ICMP, IGMP is considered part of the IP layer.

igmp header

There are different versions of Internet Group Management Protocol ( IGMP ) .IGMPv1 query / response model = age out mechanism, not responding to queries (latency 3 minutes). IGMPv2 introduction of join / leave messages, group specific queries .IGMPv3 source specific query can contain several responses in one report , join and leave for different streams can be in the same message.

igmp membership

IGMPv1 (RFC1112)

  • IGMPv1 = query / response model
  • Age out mechanism
  • No “leave”-messages.
  • (latency 3 minutes)

igmpv1 packet

IGMPv2 (RFC2236)

  • Leave messages
  • Group specific queries

igmpv2 packet

IGMPv3 (RFC 3376)

  • Source specific queries
  • Several records per report ( IP SA can be )
  •  Routers must accept this!
  • Info per group record: ( record type:– current state (include /exclude mode– changes (source /mode)
  • MC group address
  • Source IP-addresses (source = rendez vous point )

igmpv3 packet

igmp query response

igmp query response

igmp query response

igmp query response

Multicast Routing

Router to Router – PIM DM/SM, DVMRP, MOSPF etc.

Client to Router – IGMP (internet group management protocol) version 1,2,3

Multicast Control Termination (MCT) – entity which terminates the IGMP signaling which is used to initiate channel zapping; this protocol is initiated within the customer device (usually a STB) when channel changes are requested

Multicast Engine (ME) – entity which actually copies the stream in order that multiple users can view the same content. Depending upon the architecture the ME function may be done in more than one place.

 imgp topology

Static MC stream : MC stream sent/available on switch no matter if there is a subscriber or not

 Dynamic MC stream :MC stream sent to the switch only when there is a subscriber for it.
 igmp stream


L2 boxes like switches are basically unaware of L3

Multicast streams travel through as broadcast traffic (because switches can’t learn multicast addresses (GDA)). Adding IGMP (L3) intelligence to switch will inhibit broadcast Switch will forward multicast stream only to those ports on which it received an IGMP Join message.

IGMP snooping : Monitoring passively the IGMP messages passing by and then taking appropriate actions for setting up multicast branches IGMP proxy Messages do not pass transparently through .

IGMP proxy : Acts as a router and can terminate and generate IGMP messages.

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