Subject: [magma] Report on an operational problem with IGMP snooping switches.

From: "Dave Thaler" <[email-masked]>
Date: Wed, 12 Mar 2003 12:20:36 -0800

Recently, we've seen a number of instances of people
turning on IGMPv3 in their routers, while having 
IGMPv2-only switches on the link.  The IGMPv3 queries 
go out, and make it through the switch. The hosts then 
correctly respond with v3 reports.  However, since
the switch doesn't understand IGMPv3, depending
on the implementation the reports either don't make 
it back to the router, or they do but the switch blocks 
the stream since it doesn't know about the memberships, 
or whatever else.  In any case, things don't work right.

This problem is alluded to in draft-ietf-magma-snoop-06.txt
which says:
   The switch that supports IGMP snooping must flood all
   unrecognized IGMP messages to all other ports ...
[...]
   If a switch receives a non-IGMP IPv4 multicast packet without
   having first processed Membership Reports for the group
   address, it may forward the packet on all ports but it must
   forward the packet on router ports.  A switch may forward an
   unregistered packet only on router ports, but the switch must
   have a configuration option that suppresses this restrictive
   operation and forces flooding of unregistered packets on all
   ports.

   In an environment where IGMPv3 hosts are mixed with snooping
   switches that do not yet support IGMPv3, the switch's failure
   to flood unregistered streams could prevent v3 hosts from
   receiving their traffic.

I think the second paragraph won't help, however, if there is a 
mix of IGMPv2 and IGMPv3 hosts on the LAN, since the switch would 
treat the group as registered.

The current operational solution for is to
configure the router to only send IGMPv2 queries 
on a link with non-IGMPv3 switches.  If the switches 
and routers are owned by different organizations, 
however, this can be a pain.

I'd be happy to give a 5 minute presentation on this at 
IETF and facilitate a discussion on what the Magma and/or 
MBoneD WGs should do, if anything.  This problem has been 
reported by multiple customers using different switches, 
so I'd like to see this more explicitly discussed in some 
document since it's getting to be a FAQ for us.

-Dave

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