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Configuration Guide - IP Multicast

CloudEngine 8800, 7800, 6800, and 5800 V200R005C00

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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MLD Snooping

MLD Snooping

Fundamentals

MLD snooping is a basic IPv6 Layer 2 multicast function that forwards and controls multicast traffic at the data link layer. When a Layer 2 device running MLD snooping receives MLD protocol packets exchanged between downstream hosts and the upstream router, it analyzes information carried in the packets and maintains a Layer 2 multicast forwarding table accordingly. Based on the Layer 2 multicast forwarding table, the Layer 2 device forwards multicast data packets to hosts requiring the data over the data link layer.

Figure 10-2 shows how MLD snooping works in an IPv6 PIM network. When the L2 switch at the edge of the access network receives multicast data packets from the router, the switch forwards the packets to receiver hosts so that users can watch the programs they have requested. If the switch does not run MLD snooping, it broadcasts multicast data packets at Layer 2. After MLD snooping is enabled, the switch forwards multicast data packets only to specified hosts.

With MLD snooping enabled, the switch creates and maintains a Layer 2 multicast forwarding table based on information (such as packet type, multicast group address, and inbound interface) carried in MLD protocol packets exchanged between the router and hosts. In this way, multicast data can be forwarded to hosts requiring the data over the data link layer.

Figure 10-2 Multicast packet transmission before and after MLD snooping is enabled on a Layer 2 device

Concepts

In Figure 10-3, the Layer 3 router is connected to a multicast source. Layer 2 SwitchA and SwitchB run MLD snooping. Hosts A to C are receiver hosts (multicast group members).

Figure 10-3 MLD snooping ports

Figure 10-3 shows MLD snooping ports, and the following table describes these ports.

Table 10-1 MLD snooping port roles

Port Role

Function

Generation

Router port

NOTE:

In MLD snooping, a router port refers to a port on a Layer 2 device, but not a port on a router.

A router port is located on a Layer 2 device and connected to a Layer 3 multicast device, such as a designated router (DR) or MLD querier. The Layer 2 device receives multicast data packets through the router port.

  • A dynamic router port is generated by a protocol. A port becomes a dynamic router port when it receives an MLD Query message or IPv6 PIM Hello message with any source address except 0::0. IPv6 PIM Hello messages are sent by a Layer 3 multicast device from a PIM interface to discover and maintain neighbor relationships.

  • A static router port is manually configured.

Group member port

A group member port (member port for short) is receiver-host-side port on a Layer 2 device. The Layer 2 multicast device sends multicast data packets to receiver hosts through such member ports.

  • A dynamic member port is generated by a protocol. A Layer 2 device identifies a port as a dynamic member port of a group when the port receives an MLD Report message of this group.

  • A static member port is manually configured.

Router ports and member ports are an important element in Layer 2 multicast forwarding entries: outbound interface. 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 has other two elements: multicast group address and VLAN ID.
  • A multicast group address can be a multicast IP address or a multicast MAC address mapped from a multicast IP address. 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 addresses. The IP address-based forwarding mode can prevent this problem.
  • A VLAN ID specifies a Layer 2 broadcast domain.

Implementation

A Layer 2 device running MLD snooping processes various MLD messages in different ways and creates Layer 2 multicast forwarding entries based on the messages.

Table 10-2 MLD message processing by an MLD snooping device

MLD Phase

Received Message Type

Processing Method

General query

An MLD querier periodically sends MLD General Query messages to all hosts and routers (FF02::1) on the local network segment to check which multicast groups have members on the network segment.

MLD General Query message

A Layer 2 device forwards MLD General Query messages to all ports in the local VLAN, excluding the port that has received the messages. The device processes the message receiving port as follows:
  • If the port is in the router port list, the device resets the aging timer of this dynamic router port.
  • If the port is not in the router port list, the device adds it to the list and starts the aging timer.
NOTE:

The aging time of a dynamic router port defaults to 180 seconds and is configurable.

Membership report

Report messages are sent in two scenarios:
  • When receiving an MLD General Query message, a member responds with an MLD Report message.
  • A member sends an MLD Report message to the MLD querier to announce its joining to a multicast group.

MLD Report message

A Layer 2 device forwards an MLD Report message to all router ports in a VLAN. The device obtains the multicast group address from the Report message and performs the following operations on the port that has received the message:
  • If the multicast group matches no forwarding entry, the 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 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 device resets the aging timer.
NOTE:

Aging time of a dynamic router port = Robustness variable x General query interval + Maximum response time for General Query messages

Leave of group members

This phase is completed in two steps:
  1. A group member sends an MLD Done message to notify the multicast router that it has left the group.
  2. When the MLD querier receives the Done message, it obtains the multicast group address and sends an MLD Last Listener Query or Multicast-Address-and-Source-Specific Query message to the multicast group through the port that has received the Done message.

MLD Done message

The Layer 2 device determines whether the multicast group matches a forwarding entry and whether the message receiving port is in the outbound interface list.
  • If the multicast group matches no forwarding entry or the port is not in the outbound interface list of the matching entry, the Layer 2 device drops the MLD Done message.
  • If the multicast group matches a forwarding entry and the port is in the outbound interface list, the Layer 2 device forwards the MLD Done message to all router ports in the VLAN.
Assuming that the port receiving an MLD Done message is a dynamic member port, the Layer 2 device acts as follows within the aging time of the member port:
  • If the port receives MLD Report messages in response to the Last Listener Query message, the device knows that the multicast group has other members under the port and resets the aging timer.
  • If the port receives no MLD Report message in response to the Last Listener Query message, no member of the multicast group exists under the port. Then the device deletes the port from the outbound interface list when the aging timer expires.

MLD Last Listener Query or Multicast-Address-and-Source-Specific Query message

The Layer 2 device forwards the message to all ports in the corresponding VLAN excluding the port that has received the message.

When a Layer 2 device receives an IPv6 PIM Hello message, it forwards the message to all ports in the corresponding VLAN excluding the message receiving port. The device processes the message receiving port as follows:
  • If the port is in the router port list, the device resets the aging timer of this dynamic router port.
  • If the port is not in the router port list, the device adds it to the list and starts the aging timer.
NOTE:

When the Layer 2 device receives an IPv6 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 device forwards MLD Report and Done messages to the static router port. If a static member port is configured for a multicast group, the Layer 2 device adds the port to the outbound interface list for the multicast group.

After setting up a Layer 2 multicast forwarding table, a Layer 2 device looks up outbound interfaces for received multicast data packets in the table based on destination IPv6 multicast group addresses and VLAN IDs of the packets. If outbound interfaces are found for a packet, the device forwards the packet to all the member ports of the multicast group. If no outbound interface is found, the device drops the packet or broadcasts the packet in the VLAN.

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Updated: 2019-04-20

Document ID: EDOC1100039595

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