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Lessons · Network switches · IGMP snooping

Multicast, and the switch that knows who asked

IGMP snooping listens to join messages and forwards multicast only to the ports that asked for it. Without it, a switch floods multicast like a broadcast.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

An EtherNet/IP producer multicasts every few milliseconds. Flooded to every port in the VLAN, that is every device on the cell processing traffic meant for somebody else, and devices with deadlines start missing them.

How to think about it

It is ON by default, which is the fact worth carrying. When a plant network goes strange after somebody 'tidied up', this is one of the first lines to look for in the configuration.

Worked example

enable
The # prompt.
configure terminal
Into configuration mode.
vlan 20
The cell's VLAN.
name CELL4
Named.
exit
Out.
no ip igmp snooping vlan 20
Off for that VLAN. This is the change that causes the fault, reproduced.
end
Out.
show running-config
The no line is there. A default is never printed, so a line about snooping in a config means somebody turned something OFF.
configure terminal
Back in to fix it.
ip igmp snooping vlan 20
On again, and the line disappears from the configuration because it is the default once more.
end
Out.
show running-config
No snooping line at all. That is what correct looks like here: nothing.

Your turn

Turn snooping back on for VLAN 20.

ip  snooping vlan 20

The trap

Reading a config, seeing nothing about snooping, and concluding it is off. Nothing means the default, and the default is on.

Practise IGMP snooping on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.