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Re: [magma] IGMP Snooping and IGMP proxy

Subject: Re: [magma] IGMP Snooping and IGMP proxy
From: Marshall Eubanks
Date: Tue, 28 Dec 2004 12:40:55 -0500

I think that the reasons are quite  different.

IGMP snooping is used so that, on a switched LAN, there is no need to
flood multicast packets to every device on every part of the LAN. (If
there is no snooping, there
is no way to determine where the packets should go, so they have to go

On a small LAN, or one with controlled multicasts, there may be no
need to snooping, and
multicast will work without it.

IGMP proxy is quite different. It  is used when the first 
hop router (to the receivers, not to the source) is not able or configured
to deal with multicast routing. It receives IGMP traffic,  which has a
TTL = 1, and so should not be forwarded, it keeps track of who wants
what "downstream" and regenerates the IGMP traffic to go "upstream" as
needed. It should thus be used only at the _edge_ of a switched LAN,
to talk to the upstream (if necessary).

Since the IGMP proxy is used only in cases where there is only one
path upstream, such networks are
called "stub networks" in many places.

On Tue, 28 Dec 2004 19:46:54 +0530, [email protected]
<[email protected]> wrote:
> Hi,
>   I have some doubt about IGMP snooping and IGMP proxy
> 1 .In Latest draft of IGMP snooping "draft-ietf-magma-snoop-11.txt" they
> have removed the reference of "draft-ietf-magma-proxy-06.txt" . So any
> good reason for it ?

It may be because it has expired, as of Oct 2004. It is still on the site :

> 2. Can we Implement IGMP snooping and proxy simultaneously on same
> switch ?

It seems to me that this depends.

Suppose you
have a DSLAM with interfaces A,B, C... pointing towards DSL users, and
Z pointing upstream. If DSL users are allowed to multicast to each
other, and if the DSLAM is acting as a switch to those customers, then
you might want to IGMP snoop on traffic between {A, B, C...} and proxy
between the set {A,B.C...} and Z.  I have done this on a NBMA frame
relay network.

In a switched LAN type environment, connected to a router pointing
"upstream", then the switches would IGMP snoop and the router might
IGMP proxy, but not vice versa.

Regards and Happy New Year to all.
Marshall Eubanks

> 3. Can anyone help me in describing scenario in which we should use
> snooping and in which case we should use proxy ?
> 4. While using snooping do we need to change src IP address of packet
> when we f/w report to Router port ?
> Regards
> Neeraj Gupta
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