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Configuration Guide - IP Multicast

CloudEngine 8800, 7800, 6800, and 5800 V200R005C10

This document describes the configurations of IP multicast, including IP multicast basics, IGMP, MLD, PIM (IPv4), PIM (IPv6), MSDP, multicast VPN, multicast route management (IPv4), multicast route management (IPv6), IGMP snooping, MLD snooping, static multicast MAC address, multicast VLAN, multicast network management.
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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).
IGMP Snooping

IGMP Snooping

Fundamentals

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 9-2, after receiving multicast packets from a Layer 3 device Router, Switch at the edge of the access layer forwards the multicast packets to receiver hosts. If Switch does not run IGMP snooping, it broadcasts multicast packets at Layer 2. After IGMP snooping is configured, Switch forwards multicast packets only to specified hosts.

With IGMP snooping configured, Switch 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 9-2 Multicast packet transmission before and after IGMP snooping is configured on a Layer 2 multicast device

Concepts

As shown in Figure 9-3, Router (Layer 3 device) receives multicast data from the multicast source and forwards the data to downstream devices. IGMP snooping is configured on SwitchA and SwitchB. HostA, HostB, and HostC are receiver hosts.

Figure 9-3 IGMP snooping ports

Figure 9-3 shows IGMP snooping ports. The following table describes these ports.

Table 9-1 IGMP snooping ports

Port Role

Function

Generation

Router port

Ports marked as blue points on SwitchA and SwitchB.

NOTE:

A router port is a port on a Layer 2 multicast device and connects to an upstream multicast router. "Router port" mentioned in this document does not refer to a port on a router.

A router port receives multicast packets from a Layer 3 multicast device such as a designated router (DR) or IGMP querier.

  • A dynamic router port is generated by IGMP snooping. A port becomes a dynamic router port when it receives an IGMP General Query message or PIM Hello message with any source address except 0.0.0.0. The PIM Hello messages are sent from the PIM port on a Layer 3 multicast device to discover and maintain neighbor relationships.

  • A static router port is manually configured.

Member port

Ports marked as green points on SwitchA and SwitchB.

A member port is a user-side port connecting to group members. A Layer 2 multicast device sends multicast data to the receiver hosts through member ports.

  • A dynamic member port is generated by IGMP snooping. A Layer 2 multicast device sets a port as a dynamic member port when the port receives an IGMP Report message.

  • A static member port is manually configured.

A router port and a member port are outbound interfaces in Layer 2 multicast forwarding entries. A router port functions as an upstream interface, whereas a member port functions as a downstream interface. Port information learned through protocol packets is saved as dynamic entries, and port information manually configured is saved as static entries.

Besides the outbound interfaces, each entry includes multicast group addresses and VLAN IDs.
  • Multicast group addresses can be multicast IP addresses or multicast MAC addresses mapped from multicast IP addresses. In MAC address-based forwarding mode, multicast data may be forwarded to hosts that do not require the data because multiple IP addresses are mapped to the same MAC address. The IP address-based forwarding mode can prevent this problem.
  • The VLAN ID specifies a Layer 2 broadcast domain. After multicast VLAN is configured, the inbound VLAN ID is the multicast VLAN ID, and the outbound VLAN ID is a user VLAN ID. If multicast VLAN is not configured, both the inbound and outbound VLAN IDs are the ID of the VLAN to which a host belongs. For details about multicast VLAN, see Understanding Multicast VLAN Replication.

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 9-2 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. The Layer 2 multicast device processes the receiving port as follows:
  • If the port is not in the router port list, the Layer 2 multicast device adds it to the list and starts the aging timer.
  • If the port is included in the router port list, the Layer 2 multicast device resets the aging timer of the router port.
NOTE:

By default, the Layer 2 multicast device sets the aging time to 180 seconds when the router port receives an IGMP General Query message. You can set the aging time using a command.

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 router ports in a VLAN. 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 a group member port receives an IGMP Report message, its aging time is calculated using the following formula:

Aging time = Robustness variable x General query interval + Maximum response time for General Query messages

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 determines whether the multicast group matches a forwarding entry and whether the port that receives the message is in the outbound interface list.
  • If no forwarding entry matches the multicast group or the outbound interface list of the matching entry does not contain the receiving port, the Layer 2 multicast device drops the IGMP Leave message.
  • If the multicast group matches a forwarding entry and the port is in the outbound interface list, the Layer 2 multicast device forwards the IGMP Leave message to all router ports in the VLAN.
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 a group member port receives an IGMP Leave message, its aging time is calculated using the following formula:

Aging time = Robustness variable x Group-Specific query interval

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.

Upon receiving a PIM Hello message, a Layer 2 multicast device forwards the message to all ports excluding the port that receives the Hello message. The Layer 2 multicast device processes the receiving port as follows:
  • If the port is included in the router port list, the Layer 2 multicast device resets the aging timer of the router port.
  • If the port is not in the router port list, the Layer 2 multicast device adds it to the list and starts the aging timer.
NOTE:

When the Layer 2 multicast device receives a PIM Hello message, it sets the aging time of the router port to the Holdtime value in the Hello message.

If a static router port is configured, the Layer 2 multicast device forwards received IGMP Report and Leave messages to the static router port. If a static member port is configured for a multicast group, the Layer 2 multicast device adds the port to the outbound interface list for the multicast group.

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 and router ports. If no outbound interface is found, the Layer 2 multicast device drops the packet or broadcasts the packet in the VLAN.

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Updated: 2019-04-20

Document ID: EDOC1100075361

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