• Wireless Network Optimization – Multicast Control Using Internet Group Membership Protocol (IGMP)

 
 
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Introduction

Multicast is often used to stream video. Multicast packets need to flood the network to reach the recipients, which means that these packets are forwarded to many network segments even though there may not be any clients on the segment that are interested in the video stream. A protocol was developed to cull this unwanted data, called Internet Group Membership Protocol (IGMP).

IGMP provides a way for a client to inform another Layer 2 device that it wants to receive a multicast stream. It does this by sending an IGMP Report with the multicast address of the session it wants to join. Layer 2 devices use IGMP Snooping to look at multicast packets and match them to a list of multicast addresses that clients have joined. IGMP and IGMP snooping provide an effective way to allow multicast packets to be pruned so they are only forwarded to ports on which clients have subscribed (IGMP Report – Join).

Arista Access Points have the ability to block unnecessary multicast traffic and convert required multicast traffic to unicast traffic, which helps to improve the wireless performance. Related article: Multicast to Unicast conversion

Use case

  • Blocks unnecessary multicast traffic on the wireless, thereby giving more airtime to other frames.
  • Unicast packets require less airtime and have reliable delivery of packets, with retries and higher rates.

Prerequisites

  • Wireless Manager and AP should be on version 8.1 or above.

How Does it Work?

IGMP provides a way for a client to inform another Layer 2 device that it wants to receive a multicast stream. It does this by sending an IGMP Report with the multicast address of the session it wants to join. Layer 2 devices use IGMP Snooping to look at multicast packets and match them to a list of multicast addresses that clients have joined. IGMP and IGMP snooping provide an effective way to allow multicast packets to be pruned so they are only forwarded to ports on which clients have subscribed (IGMP Report – Join).

Enabling IGMP Snooping for a selected SSID blocks multicast packets if no client joins the multicast group. If any client joins a multicast group, the client entry is added to a “Snoop Table” for the multicast address within that BSSID.

If any client is present in the group for a given multicast address, multicast packets will be forwarded, otherwise they will be dropped. This applies for traffic flowing from wired to wireless and traffic between wireless clients.

For multicast to be forwarded, a client must send an IGMP Report with the address of the multicast group it wants to join. The client application is responsible for sending the IGMP Report but if the client application does not support IGMP (legacy applications), IGMP snooping can still be enabled and effective. Adding an entry for the multicast address that the application uses to the IGMP Snooping Exception List will allow the multicast traffic to flow. When an address is added to the exception list, all packets are forwarded by all APs using the SSID profile, regardless if an IGMP Report to join was sent or not. A maximum of 30 multicast addresses can be added to the exception list.

When a client that is receiving multicast roams to another AP, the snoop table is forwarded and the client does not need to send a new IGMP Report.

This feature can be enabled on CloudVision Wi-Fi from Configure > Wi-Fi > SSID > Select an SSID > RF Optimization.

If any multicast addresses are added to the IGMP Snooping Exception List, packets with the same multicast addresses are not dropped, even if no client joins the multicast group and they are not converted to Unicast even if the option to Convert Multicast to Unicast is enabled.

Note: Only IPv4 addresses are allowed in the IGMP Snooping Exception List.

You can specify the timeout value within a range of 0 to 480 minutes. The default timeout value is 5 minutes. The Snoop Timeout works as follows:

  • Every client entry is updated whenever any multicast packet of the group is to be sent to downlink. If a client does not send any IGMP packet to the group for an interval specified in the timeout, then the client is removed from the group.
  • If a group receives no Multicast packet for the specified timeout period and if no client sends any join/report, then the entire group is removed
  • If all clients leave the group, then the group is removed.

IGMP Snoop Table

Feature Description Default Range
IGMP Snooping Enables IGMP Snooping Enabled NA
IGMP Snooping Exception List Allow multicast to be delivered without client sending an IGMP Report (Join) No entries 30 Max

IGMP Snooping Restrictions

Feature Restrictions
IGMP Snooping Enabled by default

Based on client IGMP Report (Join)

Enable – blocks multicast, Disable – forwards all multicast

Applies to multicast going from Ethernet to wireless

Independent of multicast/unicast conversion

Snoop table forwarded when client roams

AP does not send IGMP Query

IGMP Snoop Protected Address Max 30 multicast addresses

Internal protected addresses

224.0.0.1/24 – query for all systems

224.0.0.22/24 – IGMP v3 addresses

Not converted to unicast if Multicast/Unicast conversion enabled

All packets forwarded on match even if no client sends an IGMP Report to join

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