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Fat AP and Cloud AP V200R008C00 CLI-based Configuration Guide

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Principles

Principles

This section describes the implementation of IGMP Snooping.

IGMP Snooping

Principles

IGMP snooping is a basic Layer 2 multicast function that forwards and controls multicast traffic at Layer 2. IGMP snooping runs on a Layer 2 multicast device and analyzes IGMP messages exchanged between a Layer 3 device and hosts to set up and maintain a Layer 2 multicast forwarding table. The Layer 2 multicast device forwards multicast packets based on the Layer 2 multicast forwarding table.

As shown in Figure 7-107, after receiving multicast packets from a Layer 3 device Router, fat AP at the edge of the access layer forwards the multicast packets to receiver hosts. If fat AP does not run IGMP snooping, it broadcasts multicast packets at Layer 2. After IGMP snooping is configured, fat AP forwards multicast packets only to specified hosts.

With IGMP snooping configured, fat AP listens on IGMP messages exchanged between Router and hosts. It analyzes packet information (such as packet type, group address, and receiving interface) to set up and maintain a Layer 2 multicast forwarding table, and forwards multicast packets based on the Layer 2 multicast forwarding table.

Figure 7-107  Multicast packet transmission before and after IGMP snooping is configured on a Layer 2 multicast device

Implementation

After IGMP snooping is configured, the Layer 2 multicast device processes the received IGMP protocol packets in different ways and sets up Layer 2 multicast forwarding entries.

Table 7-59  IGMP message processing by IGMP snooping

IGMP Working Phase

IGMP Message Received on a Layer 2 Device

Processing Method

General query

The IGMP querier periodically sends General Query messages (with destination address 224.0.0.1) to all hosts and routers on the local network segment, to check which multicast groups have members on the network segment.

IGMP General Query message

A Layer 2 multicast device forwards IGMP General Query messages to all ports excluding the port receiving the messages in a VLAN.

NOTE:

By default, the device sets the aging time of the dynamic member interface to 135 seconds when receiving an IGMP General Query message. If the IGMP General Query message carries parameters, such as the robustness variable and general query interval, the aging time is calculated following the formula: Aging time = Robustness variable x General query interval + Maximum response time for General Query messages.

Membership report

Membership Report messages are used in two scenarios:
  • Upon receiving an IGMP General Query message, a member returns an IGMP Report message.
  • A member sends an IGMP Report message to the IGMP querier to announce its joining to a multicast group.

IGMP Report message

A Layer 2 multicast device forwards an IGMP Report message to all ports in a VLAN excluding the port receiving the message. The Layer 2 multicast device obtains the multicast group address from the Report message and performs the following operations on the port receiving the message:
  • If the multicast group matches no forwarding entry, the Layer 2 multicast device creates a forwarding entry, adds the port to the outbound interface list as a dynamic member port, and starts the aging timer.
  • If the multicast group matches a forwarding entry but the port is not in the outbound interface list, the Layer 2 multicast device adds the port to the outbound interface list as a dynamic member port, and starts the aging timer.
  • If the multicast group matches a forwarding entry and the port is in the router port list, the Layer 2 multicast device resets the aging timer.
NOTE:

After receiving the IGMP Report message, the device sets the aging time of the dynamic member interface to 10 minutes.

Leave of multicast members

There are two phases:
  1. An IGMPv2/IGMPv3 member sends an IGMP Leave message to notify the IGMP querier on the local network segment that it has left a multicast group.
  2. Upon receiving the IGMP Leave message, the IGMP querier obtains the multicast group address and sends an IGMP Group-Specific/Group-Source-Specific Query message to the multicast group.

IGMP Leave message

The Layer 2 multicast device forwards the IGMP Leave message to all interfaces in the VLAN regardless of whether the multicast group matches a forwarding entry.

The following assumes that the port receiving an IGMP Leave message is a dynamic member port. Within the aging time of the member port:
  • If the port receives IGMP Report messages in response to the IGMP Group-Specific/Group-Source-Specific Query message, the Layer 2 multicast device knows that the multicast group has members connected to the port and resets the aging timer.
  • If the port receives no IGMP Report message in response to the IGMP Group-Specific/Group-Source-Specific Query message, no member of the multicast group exists under the interface. Then the Layer 2 multicast device deletes the port from the outbound interface list when the aging time is reached.
NOTE:

After receiving the IGMP Leave message, the dynamic member interface is aged out immediately.

IGMP Group-Specific/Group-Source-Specific Query message

An IGMP Group-Specific/Group-Source-Specific Query message is forwarded to all ports in a VLAN excluding the port receiving the message.
NOTE:

After receiving the IGMP Group-Specific/Group-Source-Specific Query message, the device sets the aging time of the dynamic member interface to the maximum response time.

After a Layer 2 multicast forwarding table is set up, the Layer 2 multicast device searches the multicast forwarding table for outbound interfaces of multicast data packets according to the VLAN IDs and destination addresses (group addresses) of the packets. If outbound interfaces are found for a packet, the Layer 2 multicast device forwards the packet to the matching member ports. If no outbound interface is found, the Layer 2 multicast device drops the packet.

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Updated: 2019-01-11

Document ID: EDOC1000176006

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