We are replacing a Juniper EX4200 stack with a new EX3400 stack. During the migration, we setup a bridge link between the two so that things that hadn't migrated yet could talk to things that had. This worked well until we got down to the last connection, shown in the attached image. When the red connection is in place, connectivity between client-router-1 and client-router-2 works fine. When the connection to ether6 on client-router-1 is replaced with the green connection, it stops working. A key symptom is that pings from intermediate-switch to 10.0.0.1 go out interface ge-0/1/3 instead of ge-0/0/4 (determined by mirroring ge-1/2/3 on new-router and tsharking).
Arp on intermediate-switch shows the right info, but just "vlan.1210", not which physical interface the mac appears on:
> show arp MAC Address Address Name Interface Flags ec:9b:8b:21:b2:ae 10.0.0.1 10.0.0.1 vlan.1210 none e4:8d:8c:1b:ae:09 10.0.0.10 10.0.0.10 vlan.1210 none
spanning tree is disabled on intermediate-switch and on new-router ge-1/2/3, which otherwise would be the first thing that comes to mind when remote changes have local effect (we've run into that nastiness before).