Re: [v6ops] Interesting problems with using IPv6

Brian Haberman <brian@innovationslab.net> Wed, 10 September 2014 12:19 UTC

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Date: Wed, 10 Sep 2014 08:19:17 -0400
From: Brian Haberman <brian@innovationslab.net>
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To: Fernando Gont <fgont@si6networks.com>, v6ops@ietf.org
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Subject: Re: [v6ops] Interesting problems with using IPv6
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On 9/9/14 8:40 PM, Fernando Gont wrote:
> Hi, Brian,
> 
> On 09/09/2014 02:09 PM, Brian Haberman wrote:
>>> We probably use MLD because "If you use multicast, you use MLD". Truth
>>> is that, *unless your switch does MLD snooping* (and hence you *need*
>>> MLD, or else your packets will not flow around), you could completely
>>> kill MLD, and ND would still work just fine.
>>
>> Sure, since NDP is link-local.  The drawback is what happens if your
>> network is using RFC 4541 snooping that relies on seeing those MLD
>> messages to build forwarding/filtering tables?
> 
> Exactly. That's why I said "unless...".
> 
> 
> 
>>> Not to mention that there are nodes that default t running MLDv2 *for
>>> this* (way overkill, IMO)
>>
>> Why is MLDv2 overkill?
> 
> If you're just going to use MLD for ND, MLDv2 is unnecessarily complex
> -- whole different game if you're using non-link-local multicast, of course.

I disagree.  When sending MLD Reports for NDP, you don't need to involve
the complexity of MLDv2 source filters.  You simply send a group-only
Report message.

> 
> That said, I seem to recall the node reqs RFC requires MLDv1, rather
> than MLDv2?

Seems orthogonal to the discussion since MLDv2 can act in MLDv1 mode for
these link-local multicast groups.  Additionally, RFC 5790 has
simplified MLDv2 to avoid some complexities.

Regards,
Brian