IGMP Snooping

The Internet group management protocol (IGMP) Snooping technology is used to enhance the multicast management capability of Layer 2 switching equipment. This technology applies to data service boards that support Layer 2 switching. Based on the interception of Layer 3 IGMP packets, the Layer 2 multicast function is created and maintained, to prevent multicast packets from being broadcast in the Layer 2 equipment.

Overview

Restriction on the User Access: Illegal users are prevented from using multicast services by configuring the attributes of the multicast strategy, static multicast group, static router port and unknown multicast service forwarding. As shown in Figure 2, the multicast group strategy is configured on the equipment. In this way, the range of the group addresses that can be accessed by users through the access control list (ACL) is set. Thus, the multicast packets that can be received by the user host can be controlled.
Quick Response to the Link Change: When the topology of the network where the equipment resides changes, the switching equipment should set up new forwarding table items in minutes after receiving the subsequent multicast query packet. As a result, the multicast services are interrupted for a long time. The IGMP Snooping notifies the change of the response spanning tree topology and fast transmits the query packets for the router. In this way, the switching equipment in the downstream can fast learn the multicast information and decrease the service interruption time.

Basic Concepts

Enabling the IGMP Snooping Protocol
- If the IGMP Snooping protocol is enabled, the equipment begins to perform the learning and aging of the router port and multicast group. Then, the multicast services are multicast in corresponding multicast groups.
- If the IGMP Snooping protocol is disabled, the equipment stops learning and aging the router port and multicast group, and deletes all the learned dynamic multicast groups.
Router Port
The router port refers to the port that faces the multicast router. The Ethernet data board of the equipment regards the port that receives the IGMP query packets as the router port. Router ports are classified into two types:
- Dynamic router ports: They receive the IGMP query packets. These ports are decided on the basis of the packets transmitted between routers and hosts. Moreover, they are dynamically maintained. Each of this port can enable a router port aging timer. When the timer times out, this router port is invalid and the multicast group that relies on the router port is deleted.
- Static router ports: They are specified with configuration commands, and are not aged.
  NOTE:
  Currently, all the Ethernet data boards of the OptiX NG WDM equipment cannot be configured with static router ports.
Member Port
The multicast member port refers to the port that faces the host of the member. The Layer 2 equipment forwards the multicast service packets to these ports. The multicast group member ports, referred to as member ports for short, are classified into the following two types:
- Dynamic member ports, which can receive the IGMP report packets. These ports are decided on the basis of the packets transmitted between routers and hosts. Moreover, they are dynamically maintained. Each dynamic member port is aged after reaching the maximum non-response times.
- Static member ports, which are specified by users by using configuration commands, cannot be aged.
  NOTE:
  Currently, all the Ethernet data boards of the OptiX NG WDM equipment cannot be configured with static member ports.

Basic Principle

As a multicast constraint mechanism of the Layer 2 Ethernet switch, the IGMP Snooping runs at the data link layer, to manage and control multicast groups.

When the Layer 2 Ethernet switch receives an IGMP packet between the host and router, the IGMP Snooping analyzes the information carried by the packet.

When monitoring an IGMP response packet sent by the host, the switch adds the host into the corresponding multicast table.
When monitoring an IGMP departure packet sent by the host, the switch deletes the multicast table item corresponding to the host.

By monitoring IGMP packets continuously, the switch creates and maintains the MAC multicast address table at Layer 2, and forwards multicast packets sent by the router according to the MAC multicast address table.

If the IGMP Snooping does not run, multicast packets are broadcast at Layer 2. When the IGMP Snooping runs, the packets are multicast instead of broadcast at Layer 2. See Figure 1.

Figure 1 IGMP Snooping

Compliant Standard and Protocol: The IGMP complies with RFC 4541, Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches.

Application

The IGMP Snooping enhances the resource utilization through multicast forwarding, and the network security by restricting the user access.

The advantages of the application of the IGMP Snooping are as follows:

The network bandwidth can be saved.
Signals are forwarded based on the VLAN. Hence, the information security is enhanced.
Quick response is made to the link fault. Hence, the reliability is enhanced.

Figure 2 shows how to configure the IGMP Snooping function. After reaching the equipment, the multicast packets are distributed for the port of the host where group members exist in the downstream.

Figure 2 IGMP Snooping application