All things related to multicast

[Stig Venaas <>]

Dale W. Carder wrote:
> 
> Hey Tim,
> 
> On Sep 28, 2007, at 12:23 PM, Tim Stevenson wrote:
>> At 11:31 AM 9/28/2007 -0500, Dale W. Carder observed:
>>> Has anyone else seen this message from a cat6k (w/ SXF8)?
>>>
>>>  %MCAST-SP-6-GC_LIMIT_EXCEEDED: MLD snooping was trying to allocate
>>> more Layer 2 entries than what allowed (7744)
>>
>> There is a software imposed limit on MLD snooping entries in the MAC
>> table (
>> 7744 entries), this syslog is telling you that it was exceeded, which
>> means traffic for further group macs will be flooded in the vlan.
> 
> That's what I thought.
> 
>>> This was coming from a switch/router where no ipv6 multicast is routed.
>>
>> Is not routed, or there is no IPv6 multicast at all? Is MLD snooping
>> enabled or disabled on this box? MLD snooping requires a querier, but
>> does not require multicast routing.
> 
> No IPv6 at all on this box (and that side of campus.
> Any host running v6 would only have a link-local address.)

Note that all IPv6 hosts, even with just a link-local address. It will
join ff02::1 and more interestingly the so-called solicited-node
multicast address.

As an example, on this host I got link-local address
fe80::215:f2ff:fe35:307d. Then the host joined the solicited-node
multicast address based on the last 24 bytes, in this case
ff02::1:ff35:307d.

What is interesting here is that all hosts on your subnet are likely
to have IPv6 addresses that differ in the last 24 bytes, and hence all
of them will join different solicited-node multicast addresses.

This means that there can be a lot of MLD snooping state in a switch
even when hosts only have a link-local address.

I believe some MLD snooping implementations specifically ignore the
solicited-node address (or all ff02::/16) since they are generally
used for low volume signalling. I don't know exactly what Cisco does
on the different platforms though.

I don't know if this is the problem in your case, but this is
something to be aware of.

Stig

> We're just running the defaults there w.r.t. v6.  However,
> I believe that by default mld snooping is on.
> 
> One of my NOC guys is on the horn w/ TAC, and I think they
> were thinking bug.  I'll see what they find out.
> 
> Dale