Subject: [pim] IGMP sniffing Switches not compatible with PIM-SM.

From: James Courtier-Dutton <[email-masked]>
Date: Mon, 2 May 2005 11:02:34 +0100

When I say "IGMP sniffing switches", I mean switches that block all
Multicast traffic on all ports until they hear an IGMP message on that
port, at which time they open the port to specific multicast groups.

If one has the following network:

Listener -> PIM-SM Router A -> Switch B -> PIM-SM Router C -> Multicast source.

Now, the Listener might send out an IGMP Include (S,G)
the PIM-SM Router A would send a PIM-SM Join to PIM-SM Router C.
But, the Switch B will block all multicast traffic.
If PIM-SM Router A would also send a IGMP Include (S,G) to PIM-SM
Router C, then the Switch B would see it, and stop blocking the
traffic.

This would make PIM-SM much more efficient on Switched Networks.

Any comments?

James

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From: Dino Farinacci <[email-masked]>
Date: Mon, 2 May 2005 10:59:16 -0700

>> This would make PIM-SM much more efficient on Switched Networks.

    Of course the switch could listen to PIM JP messages and do the same thing
    it would have done if it heard an IGMP Include (S,G). But more efficiently
    since additional packets would not have to be sent. This technique is
    known as PIM-snooping.

Dino

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From: Marshall Eubanks <[email-masked]>
Date: Tue, 3 May 2005 12:30:05 -0400

PIM snooping is of course generally done at AS boundaries, to limit
traffic at an exchange switch.

In this example, would the PIM JP messages be propagated into the
switched LAN ? I thought in general not, so the switch has to listen to 
IGMP.

Marshall

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From: Dino Farinacci <[email-masked]>
Date: Wed, 4 May 2005 13:08:20 -0700

>> traffic at an exchange switch.

    Even more generally speaking any large fanout of routers connected through
    one or more switches could reduce replication requirements on the 
    switches.

>> switched LAN ? 

    Yes, they would since they are link-local addressed to 224.0.0.10 or
    ff02::d. But they could be limited to router-ports only after detecting
    router-ports with PIM Hello messages.

>> I thought in general not, so the switch has to listen to 
>> IGMP.

    IGMP reports are sent on router-ports as well.

Dino

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From: James Courtier-Dutton <[email-masked]>
Date: Wed, 4 May 2005 21:55:09 +0100

>     IGMP reports are sent on router-ports as well.
IGMP reports are only sent for the PIM-SM protocol multicast
addresses, 224.0.0.x
I don't think the current PIM-SM draft standard suggests using IGMP
for all the multicast streams as well.
I was suggesting that we should maybe send IGMP messages to match and
PIM-SM messages, so that older switches that only understand IGMP
could provide efficiencies.

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From: Dino Farinacci <[email-masked]>
Date: Wed, 4 May 2005 13:59:41 -0700

>> IGMP reports are only sent for the PIM-SM protocol multicast

    Right, but this has been an age long conflict because the IGMPv2 spec
    said "MAY send reports for link-local addresses".

>> for all the multicast streams as well.

    No, but no one is really suggesting that.

>> could provide efficiencies.

    Right, understood.

Dino

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From: James Courtier-Dutton <[email-masked]>
Date: Wed, 4 May 2005 22:12:56 +0100

>     said "MAY send reports for link-local addresses".

Implying that there are some implementations that don't ?

> >> could provide efficiencies.

But, do you think my suggestion is a good or bad idea? Shall we add it
to the PIM-SM draft as a MAY. ;-)

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From: Dino Farinacci <[email-masked]>
Date: Wed, 4 May 2005 14:17:36 -0700

>> Implying that there are some implementations that don't ?

    There use to be.   ;-)

>> But, do you think my suggestion is a good or bad idea? Shall we add it

    I think it is a message/bandwidth tradeoff.

>> to the PIM-SM draft as a MAY. ;-)

    No, it shouldn't go into the PIM-SM draft even if it's a good idea. I
    feel it should go in a separate draft.

Dino

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From: Marshall Eubanks <[email-masked]>
Date: Thu, 5 May 2005 00:11:06 -0400

Dear Dino;

>     feel it should go in a separate draft.

As a standalone draft ? Or as part of what other draft ?

> Dino

Regards
Marshall

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From: Dino Farinacci <[email-masked]>
Date: Wed, 4 May 2005 22:24:29 -0700

>> As a standalone draft ? Or as part of what other draft ?

    A standalone new draft.

Dino