Help With Multicasting

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Jul 27th, 2007
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Hi All,

REeeeeely trying to wrap my head around multicasting, but I am having a difficult time of it. The basic concepts are straightforward but I'm getting lost in the configurations themsevles.

I understand how to enable it, join interfaces to it, specify sparse modes and all, but then I begin to get lost.

IGMP is for switches joining groups - but why do I specify it router to router? If I put a routers interface into a mode, should it automatically pick up the flow?

For my testing I've just been using a Ghost server, sending out a multicast stream for clients - which I can see with the sh ip mroute command. I don't have any host (pc) receivers set up - will this make a difference?

I'm assuming for the exam that I just have to know the basics, but perhaps I'm confused - as likely displayed by this post.

Any tips - any simpler posting or references for it? I have Cisco Press book and their info from their site.

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Overall Rating: 5 (1 ratings)
jlhainy Sat, 07/28/2007 - 17:21
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Here is how I think of it. In order for switches to perform igmp snooping they need to know about a device to snoop on. That device is a multicast-router. When you enable ip multicast-routing on a router, and enable pim on your lan interface, the switches that are running igmp snooping now have a device to snoop on. Through igmp, when a multicast service is present, like an imaging server, that server advertises a class D address and multiple mac address are associated to that mac address. Via igmp snooping, the switches find out what mac addresses are a part of that class D address and only forward the multicast to the appropriate mac addresses in their CAM tables. It is the multicast router that povides the mapping of the class D address to the mac addresses so that multicast router is needed!

If you don't have any receiver's, then their mac addresses will not join the class D IP multicast address and nothing happens. You want to do a sh igmp groups on the multicast-router to see the actual imaging session.

In my environment, we use Novell's Zenworks for imaging. We were finding that when we imaged, the layer 2 switches treated the image as a broadcast instead of a multicast, even when igmp snooping was enabled. The network would come to a crawl! Now that I have configured the router as a multicast router, the switches can snoop on the igmp packets and now only send the multicast to where it needs to go and not everywhere. The trick was to only enable pim on the lan interface. That way, you don't have your multicast session being advertised across a WAN link.

Here is a link that may help.

Good Luck.

Jared H.

bwgray Thu, 08/09/2007 - 07:23
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Thanks Jared!

That was what I needed, it's a good document that lays it out well for understanding. It's much clearer - that and a week off in between did'nt hurt either. ;)


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