Refer to the exhibit. After these configurations were applied to routers R1 and R2. the two devices cannot form a neighbor relationship. What is the reason tor the problem?

The correct answer is C. The two routers have the same network ID. In IS-IS, each router is identified by a Network Entity Title (NET), which includes the Area ID, the System ID (6-byte unique identifier), and the NSEL byte. The System ID portion of the NET must be globally unique within the IS-IS domain - it functions like a router-level…

Core Routing

Question

Exhibit

Options

AThe two routers have the same area ID.
BThe two routers have different IS-types
CThe two routers have the same network ID
DThe two routers cannot authenticate with one another

How the community answered

(27 responses)

A
4% (1)
B
7% (2)
C
78% (21)
D
11% (3)

Explanation

In IS-IS, each router is identified by a Network Entity Title (NET), which includes the Area ID, the System ID (6-byte unique identifier), and the NSEL byte. The System ID portion of the NET must be globally unique within the IS-IS domain - it functions like a router-level address. If two routers are configured with the same NET (same System ID within the same area), IS-IS cannot distinguish between them and adjacency formation fails. Option A is incorrect because routers in the same area should share the same area ID. Option B (different IS-types) would cause adjacency issues in some cases but is not indicated here. Option D (authentication) would only apply if authentication is configured, which is not shown.

Topics

#IS-IS Adjacency#IS-IS Addressing#Network Entity Title (NET)#Troubleshooting

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