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

Question

Options

• AThe same interface cannot be advertised in two different IS-IS instances
• BThe IS-IS "ISP" and "ISP2" instances are unrelated and unable to intercommunicate
• CThe configured IS-IS NSEL value is not allowing the routing systems to establish a neighborship
• DThe interface mode ip router is-is command was not included in the script

• A
  89% (57)
• B
  6% (4)
• C
  2% (1)
• D
  3% (2)

Explanation

In Cisco IOS-XR, a single interface can only participate in one IS-IS instance at a time. If an interface is configured under two IS-IS instances simultaneously, this creates a conflict because IS-IS uses the interface to send hellos and form adjacencies, and having dual-instance membership on the same interface leads to unpredictable behavior and routing failures. This is a common misconfiguration when operators attempt to run multiple IS-IS instances and inadvertently assign the same interface to both. The IS-IS 'ISP' and 'ISP2' instances are independent routing processes that do not automatically redistribute into each other (choice B is partially true but not the 'cause of issues' in this context). The NSEL value is always 00 in standard IS-IS (choice C is a red herring). The 'ip router isis' command (choice D) is IOS syntax, not IOS-XR syntax.

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