Which of the following operations cannot control IS-IS routing?

Question

Options

• AConfigure the link priority.
• BConfigure authentication of IS-IS interface
• CConfigure IS-IS route penetration
• DConfigure the cost of the IS-IS interface

• A
  5% (2)
• B
  85% (33)
• C
  8% (3)
• D
  3% (1)

Explanation

Authentication of an IS-IS interface is a security mechanism - it verifies that neighbors are trusted before exchanging routing information, but it has no influence over how routes are calculated or which paths are preferred. It cannot manipulate topology, metrics, or route propagation, so it cannot "control" routing behavior.

• A is wrong (it can control routing): Interface priority determines which router wins the DIS (Designated Intermediate System) election on broadcast networks, directly shaping the IS-IS topology and adjacency structure.
• C is wrong (it can control routing): Route penetration (leaking routes between IS-IS Level-1 and Level-2) explicitly controls which prefixes are visible across area boundaries, directly influencing routing decisions.
• D is wrong (it can control routing): Interface cost is the primary IS-IS metric - changing it shifts traffic to higher- or lower-cost paths, which is the core mechanism for engineering routes.

Memory tip: Think of it this way - A, C, and D all change what the network looks like to the routing algorithm (topology, reachability, or cost). Authentication only controls who is allowed to speak - it's a gatekeeper, not a traffic director.

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