IPv6 /64 links (was Re: ipv6 book recommendations?)

Fri Jun 8 03:08:33 UTC 2012

On Fri, Jun 8, 2012 at 12:48 AM, Karl Auer <kauer at biplane.com.au> wrote:
> Yes - whether with ARP or ND, any node has to filter out the packets
> that do not apply to it (whether it's done by the NIC or the host CPU is
> another question, not relevant here).

It is relevant to the question of the scalability of large L2
networks.  With IPv4, ARP presents not only a network capacity issue,
but also a host capacity issue as every node expends software
resources processing every broadcast ARP.  With ND, only a tiny
fraction of hosts expend any software capacity processing a given
multicast packet, thanks to ethernet NIC's hardware filtering of
received multicasts -- with or without multicast-snooping switches.

> The original post posited that ND could cause as much traffic as ARP. My
> point is that it probably doesn't, because the ND packets will only be
> seen on the specific switch ports belonging to those nodes that are
> listening to the relevant multicast groups, and only those nodes will
> actually receive the ND packets. In contrast to ARP, which is broadcast,
> always, to all nodes, and thus goes out every switch port in the
> broadcast domain.
>
> This is pretty much the *point* of using multicast instead of broadcast.

I agree.