300-320 Question #38: Real Exam Question with Answer & Explanation

The correct answer is B: Layer 2. IS-IS (Intermediate System to Intermediate System) is unique among link-state routing protocols because it operates at Layer 2 (Data Link layer). Unlike OSPF, which encapsulates its packets inside IP (a Layer 3 protocol), IS-IS uses its own PDUs (Protocol Data Units) that run dir

Question

In which OSI layer does IS-IS operate?

Options

ALayer 1
BLayer 2
CLayer 3
DLayer 4

Explanation

IS-IS (Intermediate System to Intermediate System) is unique among link-state routing protocols because it operates at Layer 2 (Data Link layer). Unlike OSPF, which encapsulates its packets inside IP (a Layer 3 protocol), IS-IS uses its own PDUs (Protocol Data Units) that run directly over the data link layer. This means IS-IS does not rely on IP to carry its routing messages, which also makes it naturally capable of routing multiple Layer 3 protocols (IPv4, IPv6, CLNP) using a single protocol instance.

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