Is there any data on packet duplication?

William Herrin bill at
Tue Jun 23 06:53:44 UTC 2020

On Mon, Jun 22, 2020 at 10:21 PM Saku Ytti <saku at> wrote:
> On Tue, 23 Jun 2020 at 08:12, William Herrin <bill at> wrote:
> > That's what spanning tree and its compatriots are for. Otherwise,
> > ordinary broadcast traffic (like those arp packets) would travel in a
> > loop, flooding the network and it would just about instantly collapse
> > when you first turned it on.
> Metro: S1-S2-S3-S1
> PE1: S1
> PE2: S2
> Customer: S3
> STP blocking: ANY
> S3 sends frame, it is unknown unicast flooded, S1+S2 both get it
> (regardless of which metro port blocks), which will send it via PE to
> Internet.

There's a link in the chain you haven't explained. The packet which
entered at S3 has a unicast destination MAC address. That's what was
in the arp table. If they're following the standards, only one of PE1
and PE2 will accept packets with that destination mac address. The
other, recognizing that the packet is not addressed to it, drops it.

Recall that ethernet worked without duplicating packets back in the
days of hubs when all stations received all packets. This is how.

That having been said, I've seen vendors creatively breach the
boundary between L2 and L3 with some really peculiar results. AWS VPCs
for example. But then this ring configuration doesn't exist in an AWS
VPC and I've not particularly observed a lot of packet duplication out
of Amazon.

Bill Herrin

William Herrin
bill at

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