Why IGMP?
Internet Group Management Protocol (IGMP) allows switches to do what switches do best: get
the right trac to the right desnaon eciently and ‘switch’ trac away from those that don’t
need it. Although there are many devices and protocols that keep the Internet running, IGMP
can be seen as one tool to migate overburdening any one node. Without clever hardware like
routers and switches and open standards like IGMP, all the trac on the Internet would hit your
PC’s network card all the me. It’d crash for sure! Some entertainment protocols like Dante rely
on IGMP and others, like sACN, certainly can make good use of it, especially in larger systems. If
your system has more than one console or more than 20 universes of sACN, you’d do well to
educate yourself on what IGMP does and see if it makes sense to use it.
Background
IGMP allows for packet ltering and forwarding by the switch based on mulcast groups.
Mulcast trac is dened as packets with the desnaon addresses in the range of 224.0.0.0 to
239.255.255.255.
sACN is a mulcast protocol where its desnaon IP is dened by the streaming universe in the
range of 239.255.y.x, where x is the least signicant byte and y is the most signicant byte of the
16-bit universe number. For example 239.255.0.1 is sACN universe 1 and 239.255.1.1 is sACN
universe 257. Networks using sACN can take full advantage of IGMP by reducing the trac on
the link to the gateway to just the Network DMX Universes the gateway is congured to receive.
There are other entertainment protocols, such as Dante by Audinate, that require IGMP.
Note that apart from sACN (and Pathway ssACN) which both use mulcast address in the range
of 239.255.y.x, all Pathway Connecvity devices adverse they are alive and on the network by
chirping on mulcast 239.255.237.2. If you have IGMP miscongured, you may not see all
devices in Pathscape, even if you can ping them directly by their IP. (Ping is unicast). If you see
some devices on the switch local to you, but the rest of the rig connected to other switches, try
turning on IGMP Snooping and the Querier on the local switch. If more devices appear, it’s likely
the other switches have Snooping turned on and couldn’t nd a Querier.
*
To learn more about how IGMP works, watch this video.
hps://youtu.be/0MVE22JCIt4?si=7I_0X_PzFDlSsOiG
*
If your network has Layer 3 roung and is managed by an IT department, make sure the router is set up as the
Querier and not the VIAs.
IGMP Primer
IGMP Primer – November 2023
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The Mechanics
There are two mechanisms needed for IGMP to work properly:
1. First, each and every switch on the LAN (or VLAN, as is necessary when using Pathway’s
VIA switches) must enable IGMP Snooping. This means, rather than just broadcasng
the mulcast trac that comes into the switch to every other port, the switch will
actually read part of the mulcast packet, digest it and decide where it goes. This
happens VERY quickly – we call it ‘wire-speed’. The data is not routed into the
management processor to do this; it happens in the Layer 2+ switch fabric. This is one of
the reasons you have paid for a managed switch!
2. At least one switch needs to be “in charge”. It is known as the Querier. It asks who is
interested in mulcast groups and listens for end-devices wanng to join or leave said
groups. In a Pathway network, this is equivalent to a 2-Port Pathport saying that it’s
interested in, say, Univ1 and Univ2
†
. The Snooping switches listen to (or snoops on) this
conversaon and registers the port that the device is connected to. Then if it sees trac
desne for 239.255.0.1 or 239.255.0.2, it send it on, through as many switches and as
many ports needed to connect the source of the sACN to the Pathport. It also sends the
sACN to the Querier.
In pracce, it’s always good to make the Querier a switch that is likely to have the most up-me
and close to the source of the sACN. i.e., the one in the booth that is on a UPS. Choose one with
gigabit or beer capabilies as it will see all sACN trac on your network.
‡
You should also have
an addional Querier as a backup. It maers not which two VIAs you choose or what their IP
addresses are; there is a background elecon process and they will gure it out among
themselves.
Depending on the vintage of the Pathway Connecvity device, some navely use IGMPv2 and
others IGMPv3. Version 3 Querying is backward compable to v2 by design, but be aware, some
third party switches have advanced Querier opons to explicitly exclude the use of IGMPv2. This
opon can almost always be disabled to allow maximum compability.
Pathscape Setup for IGMP
1. VLANs must be enabled for every switch on the network – even if you’re not going to
use more than one VLAN on your network.
a. Select VLAN Cong tab
b. Press VLAN Global Properes
c. Change from VLAN Names to Advanced Properes
d. Change VLAN Support to Enabled
†
By the way, a single port Pathport can listen to a lot more than a single sACN universe. With custom patches, each
of the 512 slots can subscribe to eight dierent universes at once. So don’t be surprised if a 1-Port Pathport
subscribes to mulple IGMP Groups.
‡
1GB links can support 4000 universes of sACN. Whereas 100MB links saturate with about 400 acve universes.
10GB links support upward of 40,000 universes.
IGMP Primer – November 2023
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e. A sensible range is 1 through 10. The more you use, the slower the boot process.
Pracce shows that anything over 30 is excessive.
f. It’s normal to keep the Management VLAN on VLAN 1. The VIA port your Pathscape
laptop is connected to must be on the Management VLAN or you won’t be able to
manage the VIAs.
§
2. If you want, name your VLANs. This is normally done by department. Just keep in mind,
a VLAN is a Virtual Local Area Network. One device on VLAN1 will never see another
device on VLAN2.
3. While staying in the VLAN Cong tab, expand the VLAN you want to enable IGMP on:
§
It is valid to put other lighng gear and even Pathway Connecvity devices like Pathports on other VLANs. Just be
sure to congure your laptop’s connecon port on the VIA closest to you to be on the right VLAN to see them. And
keep in mind, if you plug Pathscape into a port on a dierent VLAN you won’t be able to any see any of the VIAs on
the Management VLAN (it’s a dierent LAN).
IGMP Primer – November 2023
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4. Choose the primary VIA and turn on Querier and Snooping.
5. Choose one other VIA on this VLAN and enable both Snooping and Querier to congure
the backup Querier.
6. Go to every other VIA on this VLAN (shi or control click for mul select) and turn on
Snooping only:
7. Repeat for other VLANs as necessary.
IGMP Primer – November 2023
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Front Panel Setup of IGMP per VLAN
This is what an idle VIA’s display will look like. Press in the encoder to go to the main menu.
Scroll to “Advanced Sengs” and push to enter.
Enabled VLAN support by pressing the encoder and selecng ENABLE. Note, the next menu
opon “VLAN Setup” will not be shown if VLANs are not enabled. It is recommended to use
Pathscape to setup VLANs for two reasons: 1) The Pathscape Global VLAN properes makes
sure every VIA gets the same sengs, and 2) Only with Pathscape can you name the VLANs.
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Scroll to VLAN Cong / Status and push to enter.
Choose your VLAN. My main VLAN1, labeled “Shock”, is both my management VLAN for the
VIAs and where all my other Pathway Connecvity gear lives. Other VLANs have other purposes.
Push the encoder to congure the selected VLAN. Remember, IGMP is per VLAN.
Scroll to IGMP Snooping and push to enter.
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Click to change Disabled to Enabled, push, the scroll to < Back > and push.
Note the menu has now changed. Once Snooping is enabled, you may, if you wish, make this
switch a Querier. Push to change, scroll to “Enable” push, scroll to <Back> and push.
Enable the Querier on two switches, preferably both on a UPS and in dierent physical
locaons. Only one will be used. The system will automacally elect one. At present, there is no
way to know which VIA is the acve Querier.
On any switch that is Snooping, you can list the current mulcast groups this switch will pass.
This screen shot shows the menu opon under the VLAN setup, in this case Shock(1). You can
IGMP Primer – November 2023
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also scroll to an individual port and see specically the subscribed groups for that port alone.
This screen show shows sACN universe 19, 24 and 66 will ow through this VLAN (or port,
depending on which menu item chosen). When viewing the VLAN Status | Current Mulcast
Groups, clicking on an item in the list will show which ports on this switch are subscribed to that
group.
**
**
As of Pathscape v4, the reporng of mulcast groups is ONLY available on the front display.
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