Ethernet Basics - 5

IGMP

IGMP

• The Internet Group Management Protocol (IGMP) is an Internet protocol that provides a way for an Internet computer to report its multicast group membership to adjacent routers.

• Multicasting allows one computer on the Internet to send content to multiple other computers that have identified themselves as interested in receiving the originating computer's content. Multicasting can be used for such applications as updating the address books of mobile computer users in the field, sending out company newsletters to a distribution list, and "broadcasting" high-bandwidth programs of streaming media to an audience that has "tuned in" by setting up a multicast group membership.

• Using the Open Systems Interconnection (OSI) communication model, IGMP is part of the Network layer. IGMP is formally described in the Internet Engineering Task Force (IETF) Request for Comments (RFC) 2236.

IGMP- How it works

• There are 3 important parts of an IGMP network:– Producer- The source of the multicast traffic– Consumer- The intended destination of the multicast traffic– Querier- The holder of the IGMP tree and the device that authorizes the

consumers to be able to see the produced traffic. This is done by the use of IGMP membership queries and membership reports.

– The Querier sends membership queries, the registered end users reply with membership reports

• There are 3 versions of IGMP:– V1- allows consumers to JOIN a multicast group– V2- allows consumers to Join and Leave when they are done receiving the data– V3- Allows source filtering to keep out unwanted multicasts

• IGMP snooping- Since IGMP is primarily a layer 3 protocol, layer 2 switches did not know what to do with a multicast packet, it just flooded them like broadcasts.

IGMP- How it works

• IGMP snooping allows the switch to pay attention to the IGMP membership queries and reports. By doing this the switch can register the consumers to the appropriate port and prevent the multicasts from going to ports were there are no consumers.

• Sample IGMP group ID: 239.192.55.94 (it is a Class D address)

• Sample IGMP MAC address: 01-00-5e-aa-11-d3 (01-00-5e is always used to indicate a multicast MAC address)

• Ethernet Direct switches support both snooping and query V1 and V2

IGMP

Multicast source

Router (Querier)

Multicast traffic Multicast traffic

Multicast traffic

Multicast traffic

Multicast traffic

Only these two PLCs actually want the

Multicast data

IGMP snooping turned off

• Switches eavesdrop (snoop) on the IGMP conversation between end device and querier

• Switches are able to learn which end devices want the multicast data

Multicast source

Router (Querier)

Multicast traffic Multicast traffic

IGMP Snooping on

IGMP Snooping on

IGMP- Solution

IGMP

• PLCs are notorious multicast generators• Use IGMP for I/O over ethernet applications• Use IGMP to limit multicasts from any device

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