Default behavior of the SRX when a device is connected to an interface with no IP address assigned

When a device is connected to an interface on a Juniper SRX (or similar Junos-based device) and no IP address is assigned to that interface, the default behavior depends on the specific configuration and the type of interface. Here's a breakdown of what typically happens:

1. Interface is administratively up but unconfigured:

   • No IP Address Assigned: If an interface on the SRX device has no IP address configured (either statically or via DHCP), the interface can still be in an "up" state, but it won't have any Layer 3 (IP) connectivity. The device won't be able to route or communicate with IP addresses through that interface unless the IP address is configured.
   • Interface Still Active: The physical interface can still be active (i.e., the link is up) if the physical connection is valid and the interface is not administratively disabled (shutdown). However, without an IP address, the SRX cannot process or route IP traffic through that interface.

2. Interface Configuration and Traffic:

   • If the interface is configured as an unnumbered interface (e.g., used in bridge domains or for other purposes that don't require an IP address), the SRX can still forward traffic through that interface.
   • If the interface is used as a Layer 2 interface (e.g., part of a bridge or for switching), the SRX does not require an IP address to forward traffic at Layer 2.

3. Routing and Security:

   • If you're using the SRX for routing or security policies, the lack of an IP address on an interface typically means that traffic cannot be routed to/from that interface. For example, if the interface is part of a routing instance, the absence of an IP address would prevent the SRX from using that interface to route traffic.
   • In the context of security policies, if the interface is not in the same security zone as the source/destination of traffic, even if the IP address is configured, you would need to ensure the correct security zone and policies are in place to allow communication.

4. Traffic Handling:

   • If an interface has no IP address, the SRX will not process Layer 3 traffic through that interface. However, if the interface is part of a security zone, the SRX might still inspect or apply policies for traffic entering or exiting that zone, depending on the security policy configuration.

Without an IP address on an interface, the SRX will not be able to route or assign any Layer 3 behavior to that interface, but it can still function at Layer 2 (e.g., as part of a switch or bridge). The specific handling depends on the configuration (e.g., whether the interface is part of a bridge domain, a Layer 2 switch, or if routing is involved).