Exam 3 — Study Guide

Date: November 20, 2025

Network Layer

Routing Processor

• Executes routing protocols (OSPF, BGP, RIP, etc.).
• Maintains the routing table and forwarding information base (FIB).
• Handles control-plane tasks such as topology computation and route updates.

Switch Fabric

• Internal network that transfers packets from input ports to output ports.
• Types of switch fabrics:
  • Memory-based
  • Bus-based
  • Crossbar-based
• Understand how switch fabric design affects throughput and performance.

Input and Output Ports

• Input Ports:
  • Perform physical-layer and link-layer processing.
  • Carry out forwarding table lookups.
  • May include input buffering (susceptible to Head-of-Line (HOL) blocking).
• Output Ports:
  • Handle scheduling, buffering, and transmission onto the outgoing link.
  • Implement output queueing and packet scheduling algorithms.

IPv4 Fragmentation

• Fragmentation occurs when a packet exceeds the Maximum Transmission Unit (MTU) of the outgoing link.
• Relevant IPv4 header fields:
  • Identification
  • Flags (DF = Don’t Fragment, MF = More Fragments)
  • Fragment Offset
• Key concepts:
  • Fragmentation can be done by routers or hosts.
  • Reassembly happens only at the destination.
  • Fragmentation can cause inefficiency and increased packet loss impact.

Intra-AS vs. Inter-AS Routing

• Understand the distinction between:
  • Intra-AS routing (Interior Gateway Protocols such as OSPF, RIP): routing within a single Autonomous System.
  • Inter-AS routing (Exterior Gateway Protocols such as BGP): routing between Autonomous Systems.
• Be able to explain why both types are required and how they support scalable global routing.

Traceroute and ICMP

• Know how Traceroute discovers the path to a destination by manipulating TTL values.
• Understand how ICMP Time Exceeded and Echo Reply messages are used in the process.
• Be prepared to describe the purpose of each step in Traceroute’s operation.

Routing / Forwarding Table Construction

• Understand how to construct a forwarding table, including:
  • Destination networks or prefixes
  • Next-hop router
  • Metrics or costs
• Be familiar with how routers use this information to forward packets.

Routing Algorithms

Link-State Routing (Dijkstra’s Algorithm)

• Know the key steps of Dijkstra’s algorithm for computing shortest-path trees.
• Understand the role of the Link-State Database (LSDB).
• Know how routers exchange topology information using Link-State Advertisements (LSAs).

Distance Vector Routing (Bellman-Ford Algorithm)

• Understand how the Bellman-Ford algorithm updates distance vectors and converges.
• Be prepared to explain:
  • How routers share distance vector information
  • Convergence behavior
  • Limitations such as the count-to-infinity problem

Resources

• Dijkstra’s Algorithm
• CS360: Bellman–Ford Algorithm
• CS360: Dijkstra’s Algorithm

Link Layer

Services Provided by the Data Link Layer

Be able to list and explain key services:

• Framing
• Error detection (e.g., checksums, CRC)
• Error correction (e.g., forward error correction)
• Medium access control (MAC) for shared channels

Exam Guidelines

• Format: Open-book, open-notes
• Duration: Full class period
• Content: May include a programming component
• Total Exams: 4 (the lowest score is dropped)
• Make-Up Policy:
  • Requires prior approval unless due to a documented emergency or illness

Good luck with your exam prep!