Subject: [magma] RE: Igmp snooping on small network

From: "Kimball, Karen E" <[email-masked]>
Date: Fri, 30 Apr 2004 09:26:07 -0700

Hello, Charlie,

> -----Original Message-----
>    1. Re: Question concerning IGMP snooping (Charlie Wood)
> 
> On Fri, 23 Jan 2004 09:22:23 Gorry Fairhurst wrote:
> > OK, so why allow static configuration?....
> > <snip>
> > (ii) So that I can force my switch to correctly send the IGMP
> > membership reports to *all* the upstream switches/routers - given
> > many switches don't have an upgrade path, this gives an 
> opportunity to
> > "fix" a "broken" switch/router behaviour.
> 
> On a related (and maybe clueless) note...
> 
> Given a simple network with two switches, each snooping, with 
> host(s) on each switch and a link between the switches, is it 
> necessary to either
> (a) run an IGMP router on each switch to provide detection or 
> (b) manually configure the 'link' port on each switch to 
> forward IGMP membership requests? 
> 
> Put another way, can the simple two switch network above 
> 'work' without manual configuration as described when dealing 
> with switches that have snooping but no IGMP router capability?

Only if at least one switch has the ability to become IGMP 
Querier. If so, no additional configuration should be needed 
(except an IP address for the Querier switch, which is required 
in the Queries).

> Is it possible for the IGMP snooper to send out IGMP router 
> queries with no side effect other than switch detection?

Well, there is a very important and necessary side-effect: the
hosts often use the receipt of an IGMP Query as a trigger to
send out IGMP Joins, so they can maintain group membership.

An IGMP network, no matter how small, MUST have a Querier
present (even if it's only a switch Querier). This is required
in order to make the hosts cooperate with the protocol. In
addition, the other switch may not forward any Joins to the
second switch unless it detects a Querier on the connected
link. The switch is always required to forward Joins toward
routers and the Querier. But it is not desireable to forward
Joins to host ports (which is what the port will look like if
the Querier is not present).

I hope this answers your question.

Regards,
	Karen Kimball
(Please remove all email headers in replying)

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From: "Kimball, Karen E" <[email-masked]>
Date: Fri, 30 Apr 2004 09:26:07 -0700

Hello, Charlie,

Regards,
	Karen Kimball