CS/ECE 438: Communication Networks Fall 2007
Problem Set 3 Solutions
NOTE: There will be no automatic extension for this assignment. If you do not hand in the assignment
by the start of class on Oct 17, you will get no credit for the assignment.
1. Learning Switches
Refer to Figure 1 for this question. The octagons with numbers denote learning switches, with ports
labeled by small numbers, and letters denote hosts (attached to LAN segments).
(a) Suppose switching tables for all switches are initially empty, and then the following two transmis-
sion occur: AโG,DโA. The following transmission is IโD. Which hosts hear the third
transmission?
The first transmission from Ais broadcast, so each switch knows which way Ais. The transmission
from Dto Agets heard by switches 5, 7, 2, 6, 1 on its path to A.Iโs transmission to Dgets
rebroadcast by switches 3 and 4 to hosts Fand E, and then it also gets overheard by hosts B,C,
and of course D.
(b) Show the switching table at switch 1 after the three transmissions.
The switching table is:
Host Port
A 1
D 1
I 3
1
2
4
5
6
7
A
B
C
D
E
F
G
H
I
1
2
33
Figure 1: Diagram for Problem 1.
1
