Am Montag, 22. Juni 2020, 23:53:44 schrieb William Herrin:

> On Mon, Jun 22, 2020 at 10:21 PM Saku Ytti <saku@ytti.fi> wrote:

> > On Tue, 23 Jun 2020 at 08:12, William Herrin <bill@herrin.us> 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.

>

> Regards,

> Bill Herrin

They don't have to break anything or get creative , just assume vrrp between the PE Routers.

Not sure how many vendors drop by default if they are not the active router.

Regards

Karsten