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AlliedWare™ OS
How To | Configure IGMP for Multicasting on Routers and
Managed Layer 3 Switches
Introduction
Allied Telesis routers and managed layer 3 switches use IGMP—Internet Group Management
Protocol—to track which multicast groups their clients belong to. This enables them to send
the correct multimedia streams to the correct destination.
This How To Note describes basic and advanced IGMP configuration, in the following major
sections:
an overview of IGMP and definitions of some of th
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Introduction > Contents Multiple potential IGMP queriers ...................................................................................................... 15 Example ............................................................................................................................................ 15 Explanation of multiple potential IGMP queriers .................................................................... 17 IGMP proxy ..................................................
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Introduction > Products and software versions this note applies to Stopping snoopers from snooping non-IGMP messages .............................................................. 86 Example ............................................................................................................................................ 86 Preventing an All Groups entry for a port ............................................................................... 90 Controlling which addresses create A
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Introduction > Products and software versions this note applies to The following table shows the software versions and products each feature is available on. IGMP feature Software versions Products Snooping All All except AR410, AR410S, AR725 and AR745 Multiple potential queriers All All Proxy AT-8948, x900-48: 2.8.1 and later All except AT-9800 and SwitchBlade Other products: all versions Filtering 2.7.5 and later All that support this version Filtering different message types 2.8.1 and l
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IGMP overview > Queriers and Snoopers IGMP overview IGMP terms: Clients in an IP subnetwork use IGMP to indicate that they are interested in receiving a multicast. Multicast or Multicast stream IGMP then ensures that routers and switches A flow of information—usually video or forward multicast packets out the appropriate audio—that can go from one source to ports to the interested clients. many destination clients. Group IGMP is very flexible, as the examples in this A multicast strea
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IGMP overview > Messages The key differences between a network’s Querier and its Snoopers are: The Querier generates Query messages to find out which ports need to transmit each multicast stream. The Snoopers also use Query messages to find this out, but they use the Querier’s messages—Snoopers cannot create Query messages themselves. The Querier has IGMP enabled as part of its IP configuration. Snoopers do not require any configuration because snooping is enabled by default on Allied T
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IGMP overview > Choosing group addresses Choosing group addresses This section describes things you need to be aware of when choosing addresses for your multicast groups. Reserved IP IP addresses in the range 224.0.0.0-239.255.255.255 are multicast addresses, but many addresses addresses in this range are reserved. Therefore, before choosing a multicast address, you should check its status in the “Internet Multicast Addresses” document at the IANA website at www.iana.org/assignments/mult
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IGMP overview > Choosing group addresses Avoid x.0.0.y, x.0.1.y, x.128.0.y, and x.128.1.y It is particularly important to avoid using any address in the ranges x.0.0.y, x.128.0.y, x.0.1.y, or x.128.1.y (where x is 224-239 and y is 1-254). This is because x.0.0.y and x.128.0.y will map to the same multicast MAC address as 224.0.0.y. Similarly, x.0.1.y and x.128.1.y will map to the same multicast MAC address as 224.0.1.y. Most addresses in the ranges 224.0.0.y and 224.0.1.y are reserved for
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IGMP snooping > Example IGMP snooping In this example, we discuss IGMP snooping, the key to efficient multicast traffic flow in a layer 2 network. IGMP snooping is enabled by default on switch ports in Allied Telesis managed layer 3 switches and routers—it does not require any configuration. In a single-switch network, IGMP snooping makes multicasting happen with no configuration at all. All you need to do is connect your server and clients to the switch. In a multi-switch network, at lea
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IGMP snooping > Example Configure switch 1 Switch 1 is configured with IGMP, which makes it the IGMP Querier in this network. It is best practice to make the Querier the closest switch to the multicast source, and in this example switch 1 is closest. For more information about queriers see "Multiple potential IGMP queriers" on page 15. set system name="Switch 1" # VLAN general configuration create vlan=vlan100 vid=100 add vlan=100 port=1-52 # IP configuration enable ip add ip int=vlan10
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IGMP snooping > Explanation of IGMP snooping Explanation of IGMP snooping This section steps through the events that occur in a typical use of multicasting in this network: to stream multicast packets for a group. IGMP The following figure shows the process by which IGMP tracks multicast clients and ensures learning that the correct clients receive the stream. process What happens before a multicast client exists: 49 26 1. Querier starts receiving multicast stream Multicast from serv
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IGMP snooping > Explanation of IGMP snooping Using Show command output to investigate IGMP state No group In the first stage of the figure above, the multicast server is turned on and is streaming group members 224.12.13.14 to the Querier, switch 1. Switch 1 knows about the group, but has nobody interested in receiving it. You can see this by using the command show igmpsnooping on switch 1. The output of this command shows that switch 1 has an entry for the group, but no associated ports
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IGMP snooping > Explanation of IGMP snooping This output now shows two entries, one for each of the following: group 224.12.13.14 and port 3, which shows that the client is attached to the Snooper through port 3 and is listening to group 224.12.13.14. The Snooper created this entry at stage 5 in the process ("IGMP learning process" on page 11). This entry means that the Snooper forwards packets from 224.12.13.14 out port 3. All Groups and port 26, which shows that the Snooper is connect
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IGMP snooping > Explanation of IGMP snooping Finally, look at the output of the command show igmpsnooping on the Querier. Even though switch 1 is the Querier for this network instead of a Snooper, this command shows that a client for group 224.12.13.14 is reached out port 49. Manager Switch 1> show igmpsnooping IGMP Snooping ------------------------------------------------------------------------ Status ........................... Enabled Disabled All-groups ports ........ None Vlan Name (
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Multiple potential IGMP queriers > Example Multiple potential IGMP queriers To find out more about IGMP, we next investigate what happens when more than one router or switch has an IGMP configuration. RFC 2236, Internet Group Management Protocol, Version 2, says that each Layer 2 network should have only one IGMP Querier. You may configure IGMP on more than router or switch, perhaps for redundancy, but the routers and switches have a pseudo election and the device with the lower IP beco
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Multiple potential IGMP queriers > Example Configure switch 1 Switch 1 is a potential IGMP Querier. It acts as a Snooper if it is not elected as the Querier. set system name="Switch 1" # VLAN general configuration create vlan=vlan100 vid=100 add vlan=100 port=1-52 # IP configuration enable ip add ip int=vlan100 ip=172.31.0.254 mask=255.255.255.0 enable ip igmp enable ip igmp int=vlan100 # STP general configuration enable stp=default set stp=default mode=rapid set stp=default port=1 edgeport
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Multiple potential IGMP queriers > Explanation of Multiple potential IGMP queriers Explanation of Multiple potential IGMP queriers When there are no group members Switch 1 and switch 2 are both possible Queriers, and an election determines which switch becomes the actual Querier. We can see the results of the election by using the command show ip igmp on each switch. Manager Switch 1> show ip igmp IGMP Protocol ------------------------------------------------------------------------ Status
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Multiple potential IGMP queriers > Explanation of Multiple potential IGMP queriers When a client joins a group Now imagine that Client 1 sends a Membership Report to switch 2 for the group 224.12.13.14. If we check the group membership for switch 2 by using the command show igmpsnooping, we see a group entry for 224.12.13.14. Manager Switch 2> show igmpsnooping IGMP Snooping ------------------------------------------------------------------------ Status ........................... Enabled
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Multiple potential IGMP queriers > Explanation of Multiple potential IGMP queriers To see the difference between a switch acting as a Snooper and a switch acting as a Querier, compare the IGMP snooping table for switch 1 (above) with its IGMP table (below). They seem to contradict each other. The IGMP snooping table tells us that switch 1 is aware that it is receiving the group 224.12.13.14 and will send all groups (including this one) out port 49 towards the IGMP Querier, switch 2. Howe
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Multiple potential IGMP queriers > Explanation of Multiple potential IGMP queriers Manager Switch 2> show igmpsnooping IGMP Snooping ------------------------------------------------------------------------ Status ........................... Enabled Disabled All-groups ports ........ None Vlan Name (vlan id) ..... default (1) Fast Leave .............. Off Group List .............. No group memberships. Vlan Name (vlan id) ..... vlan100 (100) Fast Leave .............. Off Group List ........
