I have a single Layer 3 Siemens RX1510 network switch split into 4 separate VLANs on different subsets.

Name     |  Gateway          |  Subnet
VLAN1    |   |
VLAN2    |   |
VLAN3    |   |
VLAN4    |   |

I have equipment hooked up to each subnet that needs to broadcast and subscribe to multicast messages between each subnet. Is there a way to easily set this up so that I do not need to manually add static multicast routes for each and every VLAN? Is there a way to use IGMP or something similar to automatically handle routing of multicast data between each VLAN?

  • 1
    First, you cannot broadcast between networks. Second, you need to enable multicast routing to route multicast between networks, and that is very different than unicast routing. Unicast routing is designed to get packets to specific destinations. Multicast routing is to prevent multicast packets from going where they are not wanted. Multicast routing will require IGMP and a multicast routing protocol, e.g. PIM. IGMP will tell PIM to send multicast packets to the network where a host used IGMP to request it. – Ron Maupin Apr 27 at 17:17
  • Will IGMP work between VLANs? I did enable static multicast routing on the switch but needed to set up individual static routes for each multicast massage. i.e. - I needed to specify the source-ip, multicast-ip, in-interface, and out-interface for every single message. I was wondering if there is an easier way. Say I didn't know all multicast and source ips, isn't there an automated way to do this? I will look into PIM, but I thought I already had IGMP enabled, I'll double check that also. – Hooplator15 Apr 27 at 18:17
  • "Will IGMP work between VLANs?" No, IGMP is a communication between a host and multicast router on a network. Multicast routing is not static routing. You do not use unicast routes for multicast, you use a multicast routing protocol, e.g. PIM. IGMP talks to the multicast routing (PIM) to tell it to start sending multicast traffic for a particular group that the host wants. – Ron Maupin Apr 27 at 18:36

You'll likely want to look into something like Protocol Independent Multicast (PIM), which seems to be covered for that platform here. As Ron points out, IGMP is only managing multicast groups within a subnet. To route multicast between subnets you'll need to enable multicast routing on the platform and then, in most cases, enabling PIM on the appropriate L3 interfaces on the box.

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