Multicast Routing on a Cicso 3750

Posted on 2009-04-29
Last Modified: 2012-05-06
I've got a Cisco 3750 switch/router, first line of show version reports:

Cisco IOS Software, C3750 Software (C3750-IPSERVICES-M), Version 12.2(35)SE5, RELEASE SOFTWARE (fc1)

Ive got 3 VLANs 1, 100 and 200. I want each VLAN to act like a regular switch with respect to multicast traffic. Meaning its a broadcast until it hits the router. I also need the multicast to be routed between the VLANs as different applications on those VLAN subscribe for them. So, I turned on PIM with the following commands:

ip multicast-routing distributed

And on each VLAN interface:

ip pim sparse-dense-mode

I couldnt just enter ip multicast-routing. It told me that was an incomplete command. So now multicast packets only go to ports on the VLAN where apps have subscribed. I would like to turn off the smarts controlling the VLAN multicast traffic but keep automatic multicast routing on. How do I do that?

Question by:MassiveD
    LVL 5

    Expert Comment

    I think you did it correctly, here's a recap for each VLAN interface:
    interface interface-id
    ip pim version 2
    ip pim sparse-dense-mode
    show run
    copy run start
    Your VLAN's all have IP's assigned right?

    Author Comment

    I didn't have the ip pim version line. I have added it. The switch doesn't support version 2, so I turned on version 1. That didn't fix my problem.

    I have IP addresses assigned to each VLAN. I have ip routing enabled and unicast routing is working fine.

    I've got 2 devices on 2 ports of a single VLAN. Each one is sending out multicast traffic on One is using UDP destination port 11841 and the other is using port 11842. Neither device actually subscribes for that multicast address. As a result, neither one can see the others multicast traffic.

    My understanding of VLANs is that they are like logical switches. When I use a simple linksys switch, each app can see the other ones traffic fine. I think the cisco switch is doing multicast routing on the VLAN. So the multicast packets only go to the ports that have subscribed. I need the VLAN to be more like a regular switch. I also need to keep the ability to dynamically route multicast traffic across the VLANS.
    LVL 5

    Expert Comment

    If you are unable to issue the command ip multicast-routing globally to enable it, then you probably are not running advanced feature set on that switch. Find it and download it.
    It is called Advanced IP Services with Web Based Device Manager
    the filename is c3750-advipservicesk9-tar.122-40.SE.tar
    Just make sure your switch has at least 128MB DRAM and 16MB Flash memory.
    With this feature set, you can enable the multicast-routing globally without having to setup subscriptions. You will still have to have ip pim sparse-dense-mode enabled for each VLAN.

    Author Comment

    That file can't be installed. It says I don't have the correct feature set.

    I can issue to following command to globally enable multicast routing:

    ip multicast-routing distributed

    The problem is, that seems to effect how multicast is treated on the VLAN level too. All the applications get their multicast traffic when they join the multicast group. I've got 1 app that doesn't join a group. It relies on the VLAN to broadcast multicast traffic. This switch doesn't seem to do that once multicast routing is enabled. If the app doesn't subscribe, it doesn't get the data, even though it's on the same VLAN as the sender. That's what I am trying to fix.

    Author Comment

    Any other ideas?

    Accepted Solution

    I figured it out. I had to turn off IGMP snooping like this

    no ip igmp snooping

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