Configuring Basic IGMP Snooping Functions

Other IGMP snooping functions can be configured only after IGMP snooping is enabled globally. Other IGMP snooping functions take effect in a VLAN only after IGMP snooping is enabled in the VLAN.

By default, IGMP snooping is disabled globally on the switch.

Internet Group Management Protocol (IGMP) manages multicast group members and runs on the network segments where Layer 3 multicast devices connect to user hosts. IGMP has three protocol versions: V1, V2, and V3. You can specify an IGMP snooping version on a Layer 2 device to enable the device to process IGMP messages of specific versions. Generally, set the IGMP snooping version on a Layer 2 device to be the same as the IGMP version on the upstream Layer 3 multicast device. If IGMP is disabled on the Layer 3 multicast device, set the IGMP snooping version to be the same as or later than the IGMP version running on user hosts.

Devices in the same VLAN must run the same IGMP version. When hosts in a VLAN run different IGMP versions, configure an appropriate IGMP snooping version on the Layer 2 multicast device, to ensure that it can process IGMP messages of all the hosts.

A router port is located on a Layer 2 device and connects to an upstream Layer 3 device (a multicast router or Layer 3 switch). When IGMP snooping is enabled in a VLAN, all interfaces in this VLAN learn forwarding entries from multicast protocol packets. When an interface receives IGMP Query messages or Protocol Independent Multicast (PIM) Hello messages, the Layer 2 device sets this interface as a dynamic router port. A router port provides the following functions:

• Receives multicast data from the upstream device.
• Forwards IGMP Report/Leave messages. IGMP Report/Leave messages received in a VLAN are forwarded only to router ports in the VLAN.

A dynamic router port has an aging time. If a dynamic router port does not receive an IGMP Query or a PIM Hello message before the aging time expires, the device deletes the port from the router port list. To enable an interface to forward IGMP Report/Leave messages to the upstream querier for a long time, configure the interface as a static router port.

On a ring network running IGMP snooping, if the downstream port of a device is configured as a static router port, downstream ports of all devices on the ring network must be configured as static router ports. Otherwise, multicast traffic cannot be forwarded normally if the Layer 2 network topology changes. Therefore, it is not recommended to configure downstream ports as static router ports.

A member port is on a Layer 2 device and connects to receiver hosts which are multicast group members. The member port can be configured manually or learned dynamically by multicast protocols. When IGMP snooping is enabled in a VLAN, all interfaces in this VLAN learn forwarding entries from multicast packets. When an interface receives IGMP Report messages, the Layer 2 device sets this interface as a dynamic member port. A dynamic member port has an aging time.

If the hosts connected to an interface need to receive multicast data for a specific group or source-group for a long time, add the interface statically to the multicast group or source-group. The manually added interface is a static member port. Static member ports will not be aged out.

When IGMP snooping is enabled on a Layer 2 device, the device can listen to IGMP protocol packets exchanged between an IGMP querier and user hosts to dynamically create Layer 2 multicast forwarding entries and provide Layer 2 multicast functions.

A Layer 2 device cannot create Layer 2 multicast forwarding entries by listening to IGMP protocol packets in the following conditions, even when IGMP snooping is enabled:

• The interfaces on the upstream Layer 3 multicast device have static multicast groups configured and do not run the IGMP protocol.

• The multicast source is located on the same Layer 2 network as user hosts, and therefore no Layer 3 multicast device is required.

In either of the preceding conditions, you can configure the IGMP snooping querier on the Layer 2 multicast device. The Layer 2 multicast device then sends IGMP Query messages to user hosts on behalf of the Layer 3 multicast device.

IGMP periodically sends Query and Response messages to maintain group memberships. When multiple members join the same multicast group, they send a large number of identical Report messages to the IGMP router. When IGMPv2 or IGMPv3 hosts leave a multicast group, they send a large number of identical Leave messages. To conserve bandwidth, configure suppression for Report and Leave messages on the Layer 2 device.

After message suppression is configured, the switch forwards a Report message only when the first member joins a group or when it receives an IGMP Query message. The switch sends a Leave message only when the last member leaves the group.

By default, the switch does not check whether IGMP messages contain the Router-Alert option and sends all the IGMP messages to the upper-layer routing protocol. To improve device performance, reduce transmission cost, and enhance protocol security, configure the switch to discard IGMP messages without the Router-Alert option.

By default, the switch sends IGMP messages with the Router-Alert option.

If an upstream multicast device is a non-Huawei device and has static multicast groups configured on user-side interfaces, multicast users are not allowed to dynamically join or leave multicast groups. In this case, disable the switch from sending IGMP Report and Leave messages carrying static group addresses to the upstream device.

Run the following commands in any view to check IGMP snooping configurations and forwarding entries.