Am Montag, 22. Juni 2020, 23:53:44 schrieb William Herrin: > On Mon, Jun 22, 2020 at 10:21 PM Saku Ytti <s...@ytti.fi> wrote: > > On Tue, 23 Jun 2020 at 08:12, William Herrin <b...@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
