Forcing Red Hat Linux to IGMP Version 2

Not a very common task and one that is relatively hard to locate for administrators doing a quick search online.  In some cases it is necessary to force Linux, in this case, Red Hat Enterprise Linux, away from the default of using IGMP Version 3 to Version 2.  This is often done to support older switches for multicasting.

In order to make this change we will be editing the /etc/sysctl.conf file by adding the following lines:

net.ipv4.conf.eth0.force_igmp_version = 2
net.ipv4.conf.lo.force_igmp_version = 2
net.ipv4.conf.default.force_igmp.version = 2
net.ipv4.conf.all.force_igmp.version = 2

You can find out more about your current running IGMP configuration by using this command:

cat /proc/net/igmp

The resulting output will show the IGMP version in use for your machine’s interfaces.  In Red Hat Enterprise Linux 5 this will default to version 3.  If you want to only see the interface and IGMP version number you can simplify with this command:

cat /proc/net/igmp | grep V | awk ‘{print $2 ” ” $5}’

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