IPv6 and multicast listener discovery


Question for those more versed in IPv6 than I: Is there any harm from
dropping ICMPv6 multicast listener discovery reports in a network
which does NOT use any multicast routing (i.e. only uses multicast
which stays within the local link). I see a LOT of idle node chatter
in the form of these reports which, of course, flood every station
since they are themselves multicast. As far as I can tell they are
used only to tell a multicast router whether to repeat a particular
set of multicast packets to the instant link. Which in my network is
-never- because there are no routed multicast packets to be repeated.

Bill Herrin

If you enable MLD snooping on your switches, the switch will block multicast going out irrelevant ports. The idea is to reduce broadcast in a broadcast domain.

Are your links or hosts limited in some way or broadcast domains
of some unreasonable size? Most of the competent switching or
managed wireless products will snoop or otherwise handle this
overhead in a sane manner. Otherwise this at best would seem to
be an over-optimization.

chatter was much lower than the IPX/SAP broadcasts we had from
20-25 yrs earlier.


Thus spake William Herrin (bill@herrin.us) on Fri, Jun 04, 2021 at 02:01:19PM -0700:

Hi Dale,

Actually, I'm doing station to station encryption with macsec using
multiple SCIs at each station so there's a magnification effect of
encrypted multicast packets that the switch can't snoop even if it
wanted to -- all the intermediate equipment sees is an opaque ethernet
frame with the broadcast flag set.

Bill Herrin