Unable to route multicast from IPcam (four vlans, one switch, one router with router-on-a-stick)

Question

Okay, so I've been searching far and wide for a solution to my problem here.

I want to be able to access an IPcam through its respective software across VLANs.

Hardware: Cisco 1841 Router, firmware version 12.2, and a Cisco 2940 8-port 100 Mbps switch.

The IPcam is in its own VLAN, connected to SW1, and from SW1 to R1 (Router-on-a-stick). From what I've read, it's fairly straightforward using IGMP Snooping, to allow the multicast to cross VLANs. (I can access it through the IPcam web application, and I can ping everything back and forth). I've done some of the work with IGMP Snooping, but I can't seem to make it work. I've gotten so far as to be able to see the IGMP group mac-address, but that's where I lost the track.

I've read through Cisco's guide on how to do it, but it didn't make sense for me anymore sadly...

If anyone knows what to configure (From start to finish), so I can adjust my configuration, it would be greatly appreciated.

Answer 1

Multicast, like broadcast, is normally confined to a single VLAN. To overcome this, there is multicast routing, which is very different than your normal unicast routing.

IGMP is only part of that. IGMP is a conversation between a host and a multicast router where a host can tell the multicast router that it wants to join or leave a multicast group. This allows the router to decide if it should route traffic for a particular multicast group to that particular network. The router must be configured for multicast routing.

IGMP snooping is a method that allows a switch to spy on the IGMP conversation between a host and multicast router. The switch will use the information to decide which switch interfaces will get traffic for which multicast groups, but it is per VLAN, not across VLANs. Just as with unicast traffic, multicast traffic needs a router to get from one VLAN to another VLAN.

On your router, you will need to globally enable multicast routing:

ip multicast-routing

Then, it gets more complicated, depending on the version of multicast routing that you want to use. For something as simple as what you have, you can always forward all the multicast traffic to all the VLANs. That is dense mode. If you want to only forward traffic to a VLAN when there is a listener on the VLAN, then that is sparse mode, but it requires more thought and configuration.

To configure dense mode, you must enable it on each router interface (subinterfaces for your router-on-a-stick):

ip pim dense-mode

If your switch is configured for IGMP snooping, then you want to configure your router interfaces (subinterfaces for your router-on-a-stick) to join the multicast group(s):

ip igmp join-group <group-address>

Cisco maintains documents that describe how to configure multicast routing. For example, Cisco IOS IP Configuration Guide, Release 12.2.