Multicasting help - Sparse or Dense

Answered Question
Apr 21st, 2010

All,

I've got a situation that I'm not sure if I need to change.

I've got a 3750 stack that's using L3 interfaces. It's configured in Dense mode at the moment. There's an uplink that connects to a Dell stack for our edge switches. The Dell stack has igmp snooping configured, and I'm seeing my groups on the switch and what ports are requesting to join that multicast group. Here's the question:

I also see the group on the 3750 for my source. We had an issue where Windows 7 has a migration utility and it brought the network to its knees. According to MS, Windows 7 uses a multicasting type method to find the requesters to be updated. Anyway, my source that's seen on the core is NOT connected to the core....it's connected to the Dell stack. My concern is that if the core is routing for it, that means that traffic is going through the uplink and back to get to the requesters who are also in the edge switch. In this design, my routers don't know about multicasting because we don't truly have a need for it; I'm just trying to control the multicasts that are coming in from random applications. The core (3750) is configured in Dense, but I'm wondering if I should configure itself as an RP and reconfigure in Sparse mode or if this would even matter?

Thanks!

John

I have this problem too.
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Correct Answer by Jon Marshall about 6 years 7 months ago

j.blakley wrote:

Awesome! I'm going to set that up and let you know

Now, what happens if I need to route between vlans on the same switch....would I need pim enabled on all SVIs?

Thanks!

If you want to route between vlans on the switch then you would need to enable PIM (sparse or dense mode) on all the L3 SVIs for the vlans that you wanted to route between. And obviously you need to enable "ip multicast-routing" in global config mode.

Jon

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Jon Marshall Wed, 04/21/2010 - 09:25

John

Is the source and receivers in the same vlan ?

If so is there any reason you have configured dense-mode other than for something to make the IGMP queries ?

Edit - dense-mode floods then prunes the multicast stream whether it has been requested or not whereas sparse-mode waits for a request from a client before sending the stream so sparse-mode is definitely more efficient in that sense.

Jon

John Blakley Wed, 04/21/2010 - 09:38

Jon,

The source and receivers are in the same vlan. I didn't configure this switch, but it's something that I've always wanted to change. My main concern is that since the 3750 is routing, it's always going to be using that uplink if the source and receivers are on the edge.

So currently in dense mode, the only thing it could prune would be the uplink port which it never does because the source and destinations are on that edge switch, which is another reason why I'm not sure if changing the mode on the 3750 is even going to do anything.

I just did a saturation test using msend and mdump. I have my laptop set up as the source using msend and I have 2 other laptops spread out through the stack (one is on switch 3 on the edge, and the other is on switch 5). From another system that's in switch 2 on the stack, but not running mdump, I'm having them ping the core (3750). When I run msend to the 2 laptops that are in the edge using -5 as the option (saturates the link with one burst of 50000 800-byte packets), the system that is pinging the core spikes from 1ms to 7ms....that's not a HUGE spike as compared to the system that's running mdump and pinging the core at the same time....that system spikes from 1ms to 300ms, but it's also receiving a ton of traffic which is understandable. So, I still think the traffic is going to the core and coming back to the edge switch, BUT is that just an initial connection or is that as the traffic is streaming from the source since the 3750 is holding the (S,G) pair?

Thanks!

John

Jon Marshall Wed, 04/21/2010 - 09:47

John

My main concern is that since the 3750 is routing, it's always going to be using that uplink if the source and receivers are on the edge.

Shouldn't do because flood and prune is between routers ie. it is multicast routing but you are within the same vlan so there is no need to route he multicast stream between the source and receivers. But it may send the traffic up the uplink to the PIM enabled interface so it can be flooded to other vlans.

However if you simply need multicast ability within the same vlan then i would disable pim altogether ie. remove it from the vlan interface and use the IGMP snooping querier function on the 3750. The querier function simply makes the IGMP queries that a pim enabled interface would make so that the IGMP snooping can listen to the responses but without having to enable multicast routing and run PIM.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_46_se/configuration/guide/swigmp.html#wp1193337

Jon

John Blakley Wed, 04/21/2010 - 10:19

Okay, I did what you said and it seems to have kept bandwidth down, but I need a real test other than using msend. Do you know of any good utilities that I can use to send multicast or a way to test it for a period of time with msend? The only tests that I've found send a few bytes and that's it....maybe I could write a perl script that loops it?

Anyway, you're right, we don't have a real need for multicasting (at least what everyone is telling me). I guess if something breaks, we would've needed that mroute table after all. Also, our router doesn't have multicasting enabled on it at all, it was only the switch. I've never been able to fully tell why they configured it that way, but from what you're saying, I only need to have PIM enabled when I'm routing between routers?

Thanks!

John

P.S. How do you test multicast if you don't have a true multicasting source?

Jon Marshall Wed, 04/21/2010 - 10:34

John

You only need PIM enabled if you want to route multicast between subnets so it could be enabled on routers or L3 switches. But as you say, if you only need it within one vlan then you don't need it. Note that some switches don't support the IGMP snooping querier function, in which case you do need to enable PIM because something has to make the IGMP queries.

To test multicast i use VLC which can be used to generate the multicast stream and also to receive it -

VLC

Jon

John Blakley Wed, 04/21/2010 - 10:40

Awesome! I'm going to set that up and let you know

Now, what happens if I need to route between vlans on the same switch....would I need pim enabled on all SVIs?

Thanks!

Correct Answer
Jon Marshall Wed, 04/21/2010 - 10:57

j.blakley wrote:

Awesome! I'm going to set that up and let you know

Now, what happens if I need to route between vlans on the same switch....would I need pim enabled on all SVIs?

Thanks!

If you want to route between vlans on the switch then you would need to enable PIM (sparse or dense mode) on all the L3 SVIs for the vlans that you wanted to route between. And obviously you need to enable "ip multicast-routing" in global config mode.

Jon

John Blakley Wed, 04/21/2010 - 11:46

Jon,

The VLC tip did the trick. I was able to multicast to 3 boxes with NO latency to the core switch.

Thanks!

John

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