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 -- William Herrin bill@herrin.us https://bill.herrin.us/