A network consultant is troubleshooting IS-IS instances to identify why a routing domains is having communication problems between the two instances. Which description of the possible cause of issues in the routing domain is true?

In IS-IS, an interface on a router can be enabled for only one IS-IS process (instance) at a time. If an administrator attempts to configure the same interface to participate in a second IS-IS instance (e.g., "ISP2") when it is already part of the first ("ISP"), the configuration will either be rejected or will overwrite the original assignment. This misconfiguration prevents the interface from forming adjacencies correctly for one or both instances, leading to communication problems within the routing domain.

B. This statement describes the default behavior of IS-IS instances, not a cause of a problem. The problem arises from the failed attempt to make them intercommunicate.
C. The NSEL (NSAP Selector) value in the NET (Network Entity Title) does not prevent IS-IS adjacency formation. Adjacencies are primarily based on matching area IDs and other link parameters.
D. While a missing ip router is-is command would cause a failure, option A describes a more specific problem directly related to running multiple instances, which is the focus of the question.

1. Cisco IOS XR Routing Configuration Guide, Release 7.3.x, "Implementing IS-IS", Section: "IS-IS Multi-Instance". (This section explains that each instance runs independently and interfaces are assigned to a specific instance).

2. RFC 1195, "Use of OSI IS-IS for routing in TCP/IP and dual environments", Section 3.1, "Addressing for IP-only routers".

3. Cisco IOS IP Routing: ISIS Configuration Guide, "Configuring Integrated IS-IS", Section: "How to Configure Integrated IS-IS". (Shows interface-level commands tying an interface to a single process).