Juniper Branch SRX LACP Weirdness
Juniper SRX 300 Series firewalls may stop forwarding traffic in some situations. The firewall says it is forwarding the traffic, but it doesn’t work. Monitoring traffic looks OK, ARP entries are present, but traffic never gets to the destination, until you clear ARP. Turns out the problem comes from using LACP with fast timers and active mode. Luckily the fix is simple.
Alert: Firewall Offline
Here’s the situation we saw: Our NMS reported a Juniper SRX320 offline. All other devices at the site were still working, but the firewall was unreachable. Traffic from the firewall to the NMS goes via the firewall’s default gateway. Firewall A in this diagram was unreachable, but Firewall B was fine.
OK, what’s happening? Why is my firewall unreachable?
Firewall says its fine?
Try to ping Firewall A, no response. From the default gateway, we can see an ARP entry for the firewall, but no response to ping. We can log in to Firewall B, and we see an ARP entry for Firewall A. Crucially: we can ping Firewall A from Firewall B. Hmmm. That’s strange. Why can we ping it from one locally connected device but not another?
From Firewall B, we SSH across Continue reading
















