In Junos OS, if an IS-IS level is not explicitly configured for a specific interface, that interface inherits the level setting from the global [edit protocols isis] configuration. The default behavior for a Juniper router, when IS-IS is enabled without a specific level designation at the global level, is to function as a Level 1 and Level 2 (L1/L2) router. Therefore, since the ge-0/0/0.0 interface in the exhibit has no specific level configured, it inherits the default L1/L2 behavior and will participate in both Level 1 and Level 2 adjacencies and routing.

B. The ge-0/0/0.0 interface will act as an L2 interface only

This is incorrect. The interface would only be L2 if level 2 were explicitly configured globally or on the interface itself.

C. The ge-0/0/0.0 interface will act as an L1 interface only

This is incorrect. The interface would only be L1 if level 1 were explicitly configured globally or on the interface itself.

D. The ge-0/0/0.0 interface will not be assigned to a level

This is incorrect. An interface actively participating in IS-IS must be assigned to at least one level; it cannot be unassigned.

---

1. Juniper Networks TechLibrary, Junos OS 22.4. Configuration Statements, level (Protocols IS-IS). "If you do not include the level statement, Level 1 and Level 2 are enabled." This confirms the default global behavior. Link

2. Juniper Networks TechLibrary, Junos OS 22.4. Configuration Statements, level (IS-IS Interface). "If you do not include this statement, the levels configured at the [edit protocols isis] hierarchy level are inherited." This confirms that the interface inherits the global setting. Link

3. Juniper Networks Day One Guide. This is IS-IS, Deciphering the Seven-Part Puzzle. Page 14, Chapter 2. "By default, a Juniper Networks router is configured as a Level 1/Level 2 router." This source corroborates the default router type. Link