5.1 - Router Design Basics

Question 1

Basic Router Architecture

Answer 1

1. Receive packet
2. Look at header to determine destination
3. Look in forwarding table to determine output interface
4. Modifies header
5. Send packet to output interface

Question 2

Line Card (I/O)

Answer 2

The interface by which a router sends and receives data

Question 3

Crossbar Switching

Answer 3

Every input port has a connection to every output port

During each timeslot, each input connected to zero or one outputs

Question 4

Virtual Output Queues

Answer 4

Instead of having a single queue at input, there is one queue per output port

This prevents packets from being blocked

Question 5

Scheduling

Answer 5

During which timeslots should input and output ports be matched

Question 6

Round-Robin Scheduling

Answer 6

Given a set of queues, router services in order

Problem: Packets may have different sizes

Question 7

Bit by bit scheduling

Answer 7

During each time slot, each queue only has 1 bit serviced

Problem Feasibility

Question 8

Fair Queueing

Answer 8

Service packets according to their soonest “finishing time”

Fair Queuing algorithm computes the virtual finishing time of all candidate packets, which are packets at the head of all non-empty queues

Services queue with minimum virtual time

Question 9

Each line card has own forwarding table copy. What does this prevent?

Answer 9

Prevents a central table from becoming a bottleneck at high speeds