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I am curious as to whether udp multicast creates a lot of unnecessary traffic on an internal LAN.

I assume the obvious answer to me is yes, but is it enough to congest the switch and slow down all other traffic?

If machines are using DHCP and IP addresses are not consistently assigned to the same node, I am thinking the best way to send data is via multicast.

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  • Do your switches support IGMP snooping?
    – Ron Maupin ♦
    Apr 22, 2016 at 14:57
  • Yes, I see I could possibly use it to direct multicast only to users that want to receive them?
    – user_ABCD
    Apr 22, 2016 at 14:59
  • A switch which supports IGMP snooping can listen in on IGMP requests from hosts, and send only to those switch ports where hosts have requested to become members of the multicast group. It's a little more complicated if you have multiple switches in the same broadcast domain.
    – Ron Maupin ♦
    Apr 22, 2016 at 15:01
  • I am naive (know little about IGMP snooping), so is it much different than setting up VLAN per say and only sending my traffic within the VLAN?
    – user_ABCD
    Apr 22, 2016 at 15:04

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Not necessarily. Switches which support IGMP snooping can listen in on IGMP requests from hosts, and send only to those switch ports where hosts have requested to become members of the multicast group. It's a little more complicated if you have multiple switches in the same broadcast domain. How you configure your switch depends on the switch model.

A router will not forward multicasts unless you have configured multicast routing on the router. How you configure that is dependent on the router model.

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  • Is there a large difference between IGMP snooping and MVR? They seem similar to me.
    – user_ABCD
    Apr 29, 2016 at 13:37
  • MVR is more akin to multicast routing than IGMP snooping. IGMP is a protocol between hosts and a multicast router, and IGMP snooping lets a switch snoop on those messages in a particular VLAN. Multicast routing allows multicast to move between networks, including VLANs. MVR also allows multicast to move between VLANs, but multicast routing must be disabled.
    – Ron Maupin ♦
    Apr 29, 2016 at 14:00
  • @user_ABCD, See cisco.com/c/en/us/td/docs/switches/lan/catalyst3750/software/… "Although MVR operates on the underlying mechanism of IGMP snooping, the two features operate independently of each other. One can be enabled or disabled without affecting the behavior of the other feature. However, if IGMP snooping and MVR are both enabled, MVR reacts only to join and leave messages from multicast groups configured under MVR. Join and leave messages from all other multicast groups are managed by IGMP snooping."
    – Ron Maupin ♦
    Apr 29, 2016 at 14:01
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It depends on the switches and how they are configured.

If the switches support IGMP snooping and are configured to use it then the multicast traffic should be limited to the nodes that have joined the multicast groups.

But if the switches don't support IGMP spoofing or it is disabled then the traffic will be flooded to all ports. This could easily cause significant network problems.

Tread carefully when using multicast.

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