Analogy
Imagine you run a magazine subscription service with different magazines on different topics, such as sports, cooking, and technology. You have many customers located in different cities, and you want to make sure that each customer receives only the magazines they have subscribed to without receiving magazines they are not interested in.
To accomplish this, you could use a technique similar to IGMP snooping with a querier. You would assign a local agent to act as a "querier" for each city. The querier agent would periodically send out a message to all customers in the city, asking which magazines they are interested in receiving.
When a customer subscribes to a magazine, you would make a note of their name and the magazine they are interested in. When it's time to deliver the magazines, you would send the magazines to the customers who have subscribed to them in each city.
The querier agent would continue to periodically check which customers are interested in which magazines and update its list accordingly. This ensures that each customer receives only the magazines they have subscribed to without receiving magazines they are not interested in, even as subscriptions change over time.
Similarly, IGMP snooping with a querier allows switches to forward multicast traffic only to the ports that have hosts that have joined the multicast group, reducing unnecessary network congestion and optimizing network performance, while the querier ensures that the multicast traffic is properly managed and distributed to the appropriate hosts.
What is IGMP?
IGMP stands for Internet Group Management Protocol. It is a communication protocol used by IP hosts and adjacent multicast routers to establish multicast group memberships. When a host wants to receive multicast traffic from a specific multicast group, it sends an IGMP message to the local multicast router indicating its desire to join the group. The multicast router uses this information to forward multicast traffic to the requesting host. IGMP is primarily used in local area networks (LANs) where multiple hosts want to receive the same multicast traffic. It helps to conserve network bandwidth by ensuring that multicast traffic is only sent to hosts that have expressed an interest in receiving it. There are several versions of IGMP, including IGMPv1, IGMPv2, and IGMPv3. Each version provides additional features and improvements over the previous version.
What is Multicast?
Multicast is an efficient way of transmitting data to multiple recipients as it reduces the amount of network bandwidth required for sending data to each individual recipient. Multicast traffic is primarily used in local area networks (LANs) where multiple hosts want to receive the same multicast traffic, such as video streaming or online gaming.
Multicast is also used in some routing protocols, where it allows routers to share routing information with each other efficiently. Additionally, it is used in some IP-based applications such as video conferencing, where multiple participants can join a multicast group to receive the same video stream.
What is a Multicast Group?
A multicast group is a logical group of network devices that are interested in receiving the same data stream, typically audio or video, from a single source at the same time. Instead of sending multiple copies of the same data to each individual device separately, the data is sent once and then distributed to all devices that have joined the multicast group. This conserves network bandwidth and reduces the load on the source device and the network.
Each multicast group is identified by a unique multicast IP address, which is in the range of 224.0.0.0 to 239.255.255.255. Devices that want to receive the multicast traffic from a specific group can join the group by sending an IGMP join message to their local router or switch. Once a device joins a multicast group, it can receive the multicast traffic sent to that group's IP address.
What is an IGMP Querier?
An IGMP querier is a network device that is responsible for querying the other devices on a local network to determine which multicast groups they are interested in receiving traffic for. The querier can be a router or a switch that is configured to act as a querier.
The IGMP querier sends out IGMP query messages to all devices on the network to ask which multicast groups they want to receive traffic for. The devices that want to receive the traffic send back IGMP join messages to the querier, indicating which multicast groups they are interested in. The querier then uses this information to establish the necessary multicast distribution tree and forward multicast traffic only to the ports where the interested devices are located.
The querier also ensures that the multicast group membership information is up to date by periodically sending out IGMP queries to check which devices are still interested in which multicast groups. If a device no longer wants to receive traffic for a particular multicast group, it can send an IGMP leave message to the querier to indicate that it is no longer interested.
Overall, the IGMP querier plays a critical role in managing multicast traffic on a local network and ensuring that the multicast traffic is distributed only to the devices that are interested in receiving it, while conserving network bandwidth and reducing unnecessary network congestion.
What is IGMP Snooping?
IGMP snooping is a network feature that allows a switch to listen to Internet Group Management Protocol (IGMP) messages and forward multicast traffic only to the ports that have hosts that have joined the multicast group. This reduces unnecessary network congestion and optimizes network performance by preventing multicast traffic from being flooded to all ports on the switch.
When a switch is configured with IGMP snooping, it listens to the IGMP messages exchanged between devices on the network and creates a table that maps the multicast group IP addresses to the ports where devices that have joined the multicast group are located. The switch then forwards multicast traffic only to the ports that are interested in receiving it, based on the information in the table. This ensures that the multicast traffic is delivered only to the devices that want it, and not to all devices on the network.
IGMP snooping is typically used in conjunction with an IGMP querier, which is responsible for querying the devices on the network to determine which multicast groups they are interested in receiving traffic for. The IGMP querier sends out IGMP queries to all devices on the network, and the devices that want to receive traffic for a particular multicast group send back IGMP join messages. The IGMP snooping switch listens to these messages and updates its multicast group table accordingly.
Overall, IGMP snooping is a valuable feature for managing multicast traffic on a local network and optimizing network performance, especially in environments where multicast traffic is prevalent, such as video conferencing, IP TV, and live streaming applications.
How long does it take for IGMP Snooping to prune/stop multicast?
The time it takes for IGMP Snooping to prune multicast traffic depends on several factors, including the network topology, the number of devices on the network, and the IGMP snooping configuration.
Typically, IGMP Snooping will prune multicast traffic immediately after it receives an IGMP leave message from a device indicating that it is no longer interested in receiving traffic for a specific multicast group. This means that the switch will update its multicast group table and stop forwarding multicast traffic for that group to the ports where the device was located.
The time it takes for IGMP Snooping to prune multicast traffic can also be influenced by the IGMP Snooping timer values configured on the switch. These timers determine how long the switch will wait before removing a multicast group from its table after it stops receiving IGMP join messages from devices that were previously interested in that group.
In general, IGMP Snooping is designed to be fast and efficient and will prune multicast traffic as soon as possible after it receives an IGMP leave message from a device. However, the exact timing can vary depending on the specific network configuration and IGMP Snooping settings.
IGMP Life Cycle
- Multicast group creation: The first step in the IGMP cycle is to create a multicast group, which involves choosing a multicast group address and assigning a port number.
- Joining the multicast group: The next step is for interested hosts to join the multicast group by sending IGMP join messages to the multicast group address. The IGMP querier on the network will receive the join messages and update its multicast forwarding table to indicate that the host has joined the group.
- Establishing a connection: Once the host has joined the multicast group, it can receive multicast traffic sent to the multicast group address and port number. The sender sends the multicast traffic to the multicast group address and port number, and the network forwards the traffic only to the hosts that have joined the group.
- Leaving the multicast group: When a host is no longer interested in receiving multicast traffic for a group, it sends an IGMP leave message to the multicast group address. The IGMP querier on the network receives the leave message and updates its multicast forwarding table to indicate that the host has left the group.
- Aging out the multicast group: If there are no members in a multicast group, the IGMP querier will stop sending queries for that group after a certain time period, called the aging time. This conserves network bandwidth by preventing unnecessary queries for inactive groups.
What happens if I don't have IGMP Snooping or a Querier?
In the absence of IGMP snooping and a querier, devices on the network will not receive IGMP queries to determine which multicast groups they are interested in, and will therefore not send IGMP join messages to indicate their interest in specific multicast groups. As a result, the switch will not have information on which ports are interested in which multicast groups, and will forward multicast traffic to all ports, flooding the network with unnecessary traffic.
This can result in network congestion, high bandwidth utilization, and reduced network performance. Additionally, devices that are not interested in the multicast traffic may waste processing resources trying to process the unwanted traffic, which can lead to performance issues on those devices as well.
Overall, it is important to use IGMP snooping and an IGMP querier to manage multicast traffic on a local network and ensure that multicast traffic is delivered only to the ports where devices that have joined the multicast group are located. This helps optimize network performance and reduce unnecessary network congestion.