[j-nsp] EVPN all-active toward large layer 2?

[Rob Foehl]

I've been experimenting with EVPN all-active multihoming toward some large 
legacy layer 2 domains, and running into some fairly bizarre behavior...

First and foremost, is a topology like this even a valid use case?

EVPN PE <-> switch <-> switch <-> EVPN PE

...where both switches are STP root bridges and have a pile of VLANs and 
other switches behind them.  All of the documentation seems to hint at 
LACP toward a single CE device being the expected config here -- is that 
accurate?  If so, are there any options to make the above work?

If I turn up EVPN virtual-switch routing instances on both PEs as above 
with config on both roughly equivalent to the following:

interfaces {
     xe-0/1/2 {
         flexible-vlan-tagging;
         encapsulation flexible-ethernet-services;
         esi {
             00:11:11:11:11:11:11:11:11:11;
             all-active;
         }
         unit 12 {
             encapsulation vlan-bridge;
             vlan-id 12;
         }
     }
}
routing-instances {
     test {
         instance-type virtual-switch;
         vrf-target target:65000:1;
         protocols {
             evpn {
                 extended-vlan-list 12;
             }
         }
         bridge-domains {
             test-vlan12 {
                 vlan-id 12;
                 interface xe-0/1/2.12;
             }
         }
     }
}

Everything works fine for a few minutes -- exact time varies -- then what 
appears to be thousands of packets of unknown unicast traffic starts 
flowing between the PEs, and doesn't stop until one or the other is 
disabled.  Same behavior on this particular segment with or without any 
remote PEs connected.

Both PEs are MX204s running 18.1R3-S4, automatic route distinguishers, 
full mesh RSVP LSPs between, direct BGP with family evpn allowed, no LDP.

I'm going to try a few more tests with single-active and enabling MAC 
accounting to try to nail down what this traffic actually is, but figure 
I'd better first ask whether I'm nuts for trying this at all...

-Rob