Multicast on 3750

[Phil Bradley]

I have a 3750G stack that is running IP base of IOS. This stack does inter-vlan routing between my subnets and also between another 3750 stack connected via fiber. All routing is handled by the two stacks. I do have a 2911 off of both stacks for my call manager PRI interfaces and analog lines. We are implementing Singlewire Informacast for emergency notifications. This product uses multicast to deliver the audio to all of the VoIP phones which are in two different subnets. So here are my questions?

I assume at layer 2 on the switch stacks IGMP snooping will handle the multicast on the switches layer 2 side and this is included?

The base image has PIM routing to my understanding and can this be used to route the multicast between the SVI interfaces? Im not very knowledgeable in IGMP or PIM routing so not sure how one or each would be used in my scenario.

If I need IP services to accomplish this, then could I instead somehow use the 2911 routers as my IGMP routing source? Would this require all of my subnets to go through the 2911 routers and the stacks would then essentially be layer two only?

Thanks for the feedback!

[julijime · Accepted solution]

Hi Phil,

Here is some feedback regarding your queries. 

At the Layer 2 side IGMP is responsible for registering the Multicast Receivers (hosts) in a multicast group. IGMP Snooping on the other hand is just a feature that will prevent the multicast packets to be forwarded over all the ports on a LAN segment, this feature helps to prevent the waste of resources by only sending the traffic to those specific hosts requesting the multicast feed. (These are included on your IP base license)

Now, f you're planning to have the Multicast Source and Multicast Receivers at different VLANs then you need to enable PIM. Within your license only PIM stub routing is available, meaning that you will only be able to configure PIM passive on the SVI interfaces and full PIM on the routed uplinks. You should be able to configure PIM sparse mode on the uplinks between your stacks and PIM passive on the SVis where the source and receivers will be connected.

The below links might be useful:

http://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/White_papers/mcst_ovr.html#wp1012495

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/98289-pim-stub.html

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/12-2_53_se/configuration/guide/3750xscg/swmcast.html

HTH,

Julio

[Phil Bradley]

Hi Julio,

Thanks for the info.

Does this mean that I can have a multicast sender in vlan 10 and receivers in vlan 20 and 30 using only PIM stub routing? I just would like to clarify if I can accomplish multicast with my ip base image.

Thanks,

Phil

[Phil Bradley]

Hi Julio,

So I did a little studying on multicast and now have a pretty good understanding of PIM sparse and dense, but haven't found much info on the PIM passive. It appears that this basically just tells the switch stack that any multicast traffic seen by the stack will be sent into any SVI that has passive turned on. This sounds similar to dense mode without all the control traffic. Is this correct? 

My links between the stacks are currently layer 2 mode with Svi interfaces defined for the routing. I didn't turn these into l3 interfaces because I had one vlan that I needed to pass between the stacks. I may change this at some point but until then I don't believe I can turn on sparse mode until I make it a l3 interface? Can I just turn passive on the Svi that is on both ends of the stack? I guess the negative is that I will be sending multicast across the stack even if I don't have a receiver on the other end, correct?

[Phil Bradley]

I didn't have the ip multicast-routing command available due to IOS version 12.2(35). I upgraded to IOS 12.2(55) SE11 and now have ip multicast-routing available but is only has distributed multicast. Is this the same as ip multicast-routing?