A fundamental architectural difference between OSPF and IS-IS lies in their transport mechanism. OSPF was designed specifically for the TCP/IP protocol suite and operates at the network layer. Its packets are encapsulated directly within IP datagrams, identified by IP protocol number 89.

Conversely, IS-IS was developed for the OSI protocol stack and functions at the data link layer (Layer 2). Its control messages, known as Protocol Data Units (PDUs), are encapsulated directly into Layer 2 frames (e.g., Ethernet). This design makes IS-IS independent of the network layer protocol it is routing, as it does not rely on IP for its own transport.

B. Both OSPF and IS-IS are link-state routing protocols. They both build a topological map of the network and use Dijkstra's algorithm to compute shortest paths.

C. This statement incorrectly reverses the encapsulation methods. OSPF uses IP for transport, not Ethernet directly, while IS-IS uses Layer 2 frames, not IP.

D. OSPF is a link-state protocol, not a distance vector protocol. Distance vector protocols, such as RIP, operate on different principles (e.g., Bellman-Ford algorithm).

1. Internet Engineering Task Force (IETF). (1998). RFC 2328: OSPF Version 2. Section 1.2

"Protocol overview". This document states

"OSPF runs directly over the internet protocol layer... OSPF does not use a transport protocol (such as UDP or TCP). OSPF has been assigned IP protocol number 89."

2. Internet Engineering Task Force (IETF). (1990). RFC 1195: Use of OSI IS-IS for routing in TCP/IP and dual environments. Section 3

"Encapsulation". This standard specifies

"IS-IS protocol data units (PDUs) shall be encapsulated in the data link frames... On broadcast links such as Ethernet... IS-IS PDUs are transmitted in standard LAN frames."

3. Doyle

J.

& Carroll

J. (2005). Routing TCP/IP

Volume 1 (2nd ed.). Cisco Press. Chapter 11

"IS-IS for IP Networks"

p. 638. The text explicitly contrasts the two protocols: "Whereas OSPF packets are encapsulated in IP packets

IS-IS PDUs are encapsulated directly into data-link frames." This book is a standard reference in advanced networking courses.