Subject: Re: [magma] question on igmp snooping

From: Anouk Rocher <[email-masked]>
Date: Thu, 12 Jun 2003 16:07:06 -0400

Hello Karen,

Thank you for the summary. It leads one to a model
where if an IGMP snooping network does not have a multicast
router, one or more of the IGMP snooping switches behave
like one and send out IGMP queries. This certainly gets
rid of configuring switch-to-switch ports manually for
IGMP report forwarding. However, the IGMP queriers will 
need to be configured with the list of multicast groups 
they are supposed to query for. So there is still some
manual intervention required.

Regardless, I think even without an IGMP querier the
IGMP snooping will work if switch-to-switch ports are
configured to forward IGMP reports - just not as robustly 
as witha querier. This is kinda cool as for small networks
one can get by without the complexities of PIM.

Sorry if my questions sound dumb - I used to work with DVMRP
a long time ago and am pickung up on IGMP or anything
multicast after a long hiatus.

Anouk.

---- On Thu, 12 Jun 2003, Karen Kimball
([email-masked]) wrote:
> All of this is outside of the IGMP snooping-switch document.
> It is instead part of the IGMP standard(s) itself.
> 	
> To summarize, for proper IGMP operation,
> 	* Reports and multicast traffic must be forwarded
> 		in the direction of the Querier and out all
> 		multicast router ports
> 	* There must always be an IGMP Querier present. That
> 		means that there must always be one, 
> 		preferrably more, IGMP-Querier-capable 
> 		device.
> 	
> Some IGMP snooping-switches can perform limited Querier
> capabilities, enough to make IGMP work properly. BUT, in the
> absence of a multicast router, you must be sure that at 
> least some of the IGMP switches you are using are IGMP 
> Querier-capable.
> 	
> Best regards,
> 	Karen Kimball

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---

From: Karen Kimball <[email-masked]>
Date: Thu, 12 Jun 2003 14:07:57 -0700 (PDT)

Hi, Anouk,
	
> manual intervention required.
	
This actually isn't true. The IGMP Querier sends out a 
General Query on a fixed interval, which prompts hosts to
Join any groups they are interested in.
	
You may be thinking instead of the Group-Specific Query.
This is sent out in response to an IGMP Leave message from
a host-- it is used to see if any other interested hosts
remain off of that downstream connection. So, the group to
Query for in this case was indicated by the Leaving host.

> one can get by without the complexities of PIM.
	
You really don't want to configure the switch-to-switch ports
as forward ports. It is unnecessary, and defeats the purpose
of the IGMP protocol (which is to SELECTIVELY forward 
multicast groups). You will instead be forwarding IGMP traffic
where, in some cases, you do not want it to go.
	
As I stated previously, the IGMP protocol rules for forwarding
toward the Querier/multicast routers and for requiring the
presence of a Querier on the network already take care of this.
	
For some snooping IGMP switches, such as the one I work on (is
it OK to mention this?), the configuration required is basically,
"Turn IGMP on." That's it. I believe that is true for most other
vendors, and it is certainly the goal of the draft for 
recommendations for IGMP snooping switches. 
	
I hope this clears things up. Best Regards,
	Karen Kimball

---

From: Karen Kimball <[email-masked]>
Date: Thu, 12 Jun 2003 14:07:57 -0700 (PDT)

Hi, Anouk,