An engineer is troubleshooting a failed inter-VSYS communication path between a DMZ-VSYS and an Internal-VSYS.

In a Multi-VSYS (Virtual System) architecture, Palo Alto Networks firewalls require a specific logical construct to facilitate communication that stays within the physical device. While traditional Layer 3 zones must be bound to physical interfaces, sub-interfaces, or aggregate groups,inter-VSYS communicationrelies on a specialized zone configuration known as theExternalzone type.

When traffic is routed between virtual routers using the next-vr command, the firewall needs a logical "hand-off" point to pass the session from one VSYS context to another. To achieve this, an engineer must create a zone in each VSYS and explicitly set itsType to External. These External zones do not attach to physical ports; instead, they serve as the entry and exit points for the internal backplane.

If the engineer attempts to use a standard Layer 3 zone for this purpose without an associated physical interface, the traffic will fail to egress the source VSYS or ingress the destination VSYS. Even if theSecurity PolicyandVirtual Routersettings are technically accurate, the session cannot be established because the logical path is incomplete. Therefore, assigning theExternal zone typeis a mandatory architectural requirement to bridge the gap between two logically separated virtual systems within the same hardware chassis.