We asked an expert, he doesn't know what's going on. I'm not sure even junos devs know what's up here.
We've got two EX4200, 12.3R12.4 in VCP.
Say, 2 hosts are connected to the switch. Ports (access mode) belong to single vlan.
Vlan has several subnets assigned, say 1.1.1.1/24, 1.1.2.1/24 and 1.1.3.1/24.
host1 (1.1.1.2/24) has different subnet than host2 (1.1.2.2/24). Thus, switch is going to do the routing.
host1 is sending to host2.
host2 is seeing ocassionally duplicate traffic.
host1$ ping host2
^C
210 packets transmitted, 210 received, +7 duplicates, 0% packet loss, time 62709ms
It doesn't matter icmp or not, all forwarded traffic is randomly (< 1%?) duplicated.
We enabled traffic analyzer on host1 ingress. It showed host1 is indeed sending the packets and they're not duplicated.
We enabled traffic analyzer on host2 egress. It showed host1 is sending occasionally duplicate packets.
On switch, "monitor traffic interface [host1 port] " showed (!) host1 -> host2 duplicated packets (exactly, icmp ID & SEQ matched).
This was a big WTF and probably an explanation for duplication as RE is just seeing packet not destined for switch itself and just forwards it along. And no, arp TTE was > 0, routing forwarding table was properly populated, MAC addresses learned comletely, all that.
Then, we did a trick.
assigned an address from neighbouring subnet 1.1.3.2 to host1 . Sent traffic from that IP to host2:
host1$ ping -I1.1.3.2 host2
NO DUPLICATES.
Now, would anyone explain me what magical subnet we acquired that causes the switch to react on it and randomly grab packets coming from it and try to inject into routing/control plane?
ANY suggestions are welcome. We tried everything except reboot.
THANKS.