You are configuring an IS-IS IGP network and do not see the IS-IS adjacencies established. In this scenario, what are two reasons for this problem?…

In Junos OS, establishing an IS-IS adjacency requires specific foundational configurations. First, the loopback interface (lo0) must be configured with the ISO Network Entity Title (NET) and explicitly included under the [edit protocols isis] hierarchy. If lo0 is omitted, the IS-IS process cannot identify its System ID or Area ID, completely preventing adjacency formation. Second, IS-IS pads its Hello Protocol Data Units (IIHs) to the maximum transmission unit (MTU) size to verify that the link can handle maximum-sized Link State PDUs (LSPs). The IS-IS protocol and Junos OS strictly require a minimum ISO MTU of 1492 bytes. If the underlying interface MTU is configured below 1492 bytes, the padded Hello packets are dropped in transit, causing the adjacency to fail.

Option B: IS-IS operates directly over the data link layer using ISO/CLNS framing, not IP. Therefore, IP subnet masks (such as a /30) have no impact on IS-IS adjacency formation.

Option C: Level 2 (L2) IS-IS adjacencies are explicitly designed to route traffic between different areas. Only Level 1 (L1) adjacencies require the routers to share the same area address.

1. Juniper Networks TechLibrary. Junos OS Routing Protocols Guide: IS-IS. Section: "Understanding IS-IS Hello Padding". (Details the requirement that IS-IS interfaces must support a minimum MTU of 1492 bytes for padded hellos). Document ID: Junos OS Release 23.4.

2. Juniper Networks TechLibrary. Junos OS Routing Protocols Guide: IS-IS. Section: "Configuring the IS-IS Network Entity Title (NET)". (Specifies that the NET must be applied to the lo0 interface and that lo0 must be activated within the IS-IS protocol hierarchy to establish routing). Document ID: Junos OS Release 23.4.

3. ISO/IEC 10589:2002. Information technology — Telecommunications and information exchange between systems — Intermediate System to Intermediate System intra-domain routeing information exchange protocol. Section 8.2.1 (Details Level 1 vs Level 2 adjacency area requirements, invalidating Option C).