Which statement is correct about the IS-IS ISO NET address?

In the IS-IS routing protocol, the Network Entity Title (NET) uniquely identifies the network layer of a system. A standard NET consists of an Area ID, a System ID, and a Network Selector (NSEL) which is always set to 00 for a router. The System ID portion must be strictly unique for every device within the entire IS-IS routing domain to ensure proper shortest-path tree calculations and accurate topology mapping. Consequently, the complete ISO NET address must be entirely unique for each device in the network. If duplicate System IDs (and thus duplicate NETs) are configured, the IS-IS link-state database will become corrupted, leading to severe network instability, routing loops, or traffic black holes.

A: DIS election is determined by the highest interface priority, followed by the highest MAC address (SNPA) on a LAN, not by configuring a system ID of all zeros.

C: IS-IS allows a single router to have multiple NET addresses (typically up to three in Junos) configured simultaneously to facilitate seamless area migrations without disrupting routing.

D: Level 2 (L2) routing connects different areas; therefore, L2 adjacencies successfully form between devices with different Area IDs. Matching Area IDs are strictly required only for Level 1 adjacencies.

1. Juniper Networks TechLibrary. "Understanding IS-IS ISO Network Addressing." Junos OS Routing Protocols Library, Release 23.4. Section: IS-IS NETs and System IDs. (Validates the uniqueness requirement of the NET and System ID per device).

2. Juniper Networks TechLibrary. "Configuring IS-IS." Junos OS Routing Protocols Library, Release 23.4. Section: Configuring Multiple NETs. (Validates that up to three NETs can be configured on a single Junos device for area migration).

3. IETF RFC 1195. "Use of OSI IS-IS for Routing in TCP/IP and Dual Environments." Section 3.1: IS-IS Routing. (Validates that Level 2 routers route between areas and do not require matching Area IDs, unlike Level 1).

4. ISO/IEC 10589:2002. "Information technology — Telecommunications and information exchange between systems — Intermediate System to Intermediate System intra-domain routeing information exchange protocol." Section 7.1.1: Addressing. (Defines the strict architectural requirement for System ID uniqueness across the routing domain).