That was a hard-learned lesson, yes.
@mwlucas @evilham I really hope my now-graduated students remember that. I did say ICMPv6 was absolutely required, but I wasn't funny, so I'm not sure if the message were memorable. I was almost funny talking about multicast - they should remember that!, but you can't get to multicast until the underlying network works.
As it turns out, similar problems can happen on IPv4 as they well but it's not as visible. I think it generally causes performance problems and not just outright connection drops?
ICMP (neither 4 nor 6) shouldn't ever be blocked.
@evilham Which book's this from? Networking for Systems Administrators? I don't remember it and didn't immediately find it on skip re-reading the obvious bits.
(Finding N4SA a very useful book, BTW. The earlier bits on the various protocols filled in a few gaps in my knowledge but the later bits on the various tools has been a great introduction; I hadn't tackled tcpdump before but with the knowledge there it doesn't look so bad.)
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