Review Questions

1.        List the main limitations of networks based on a shared medium

-          Uncertain part of shared bandwidth which is dedicated to a node;

-          A sharp decrease of a bandwidth dedicated to a node when a medium utilization increases above some threshold (it could happen because of increasing of a number of nodes or because of increasing of nodes traffic)

2.        Why in Ethernet networks does the sharp growth of delays start at lower values of the medium utilization coefficient than values in Token Ring and FDDI networks?

Because of a specificity of the Ethernet access method (CSMA/CD)

3.        Which are advantages of LAN switches?

4.        A switch forwarding table is created on the basis of:

A.      Source addresses - correct

B.       Destination addresses

5.        Is it possible to state that when dividing a shared medium into two segments, the load of each segment decreases twofold?

No, it is not correct because of existing of inter-segment traffic

6.        What are the negative consequences of the presence of closed loops in a network created on the basis of switches operating according to the transparent bridge algorithm?

-           Frames could be duplicated and looped;

-           The forwarding table could be in a state of permanent updating

7.        What is the goal of limiting the time-to-live period of the entries of the forwarding table?

To keep the forwarding table entries  in an updated sates that dynamically reflect all  network modifications

8.        Compare the transparent bridge and SRB algorithms.

9.        What are the purposes of user-defined filters in switches?

To restrict a reachability of some network nodes

10.     What parameters can be used when creating a user-defined filter of a switch?

-          Source and destination MAC addresses;

-          Protocol;

-          A value of any user-defined field (specified by the offset).  

11.      Can the forwarding rate exceed the filtering rate?


12.     What is a nonblocking switch?

It is a switch that can transfer frames between its ports with a rate of their arrival even though the rate is equal to a maximum value for a technology (e.g. 14880 frames per second for Ethernet or 14809 frames per second for Fast Ethernet).

13.     Is it possible for a nonblocking switch to lose packets because of queue overflow?

Yes, it can happen when traffic has an uneven distribution so that incoming frames from several ports are directed to the same output port.

14.     What mechanisms are used by switches under conditions of internal queue overflow?

·                      Traffic shaping

·                      Feedback on the basis of the PAUSE frame – correct

·                      Back pressure (artificial collisions) – correct

·                      Prioritizing

15.     Can the 10G Ethernet technology use a shared medium?


16.     Which feature of the physical interface corresponds to the number 4 in the name of the 10GBase-LX4 specification?

Data are transferred in each direction by means of 4 light waves using WDM technique.

17.     Is it possible to directly connect a LAN switch with the 10GBase-WL interface to the STM-64 port of the SDH multiplexer?

Generally no unless the SDH multiplexer has a special port 10Base-WL.

18.     Which property of LAN technologies simplifies the translation of Ethernet, Token Ring, and FDDI protocols?

A use of unique MAC address

19.     In what cases can't the FDDI frame be translated into the Ethernet frame?

If the size of a data field of FDDI frame exceeds 1500 bytes

20.     Specify the main types of switching matrix.

21.     What is the main drawback of the switching matrix?

A complexity of increasing of a port number because of BIS implementation of the switching matrix

22.     Which additional advantage is related to the use of shared memory as a switching matrix?

Buffers of the switch ports could be relatively small

23.     Why does switching on the fly have limited application in switches?

This type of switching doesn’t support technologies translation (e.g. Ethernet to FDDI) and can’t filter corrupted frames.

24.     What happens if the number of LAN addresses exceeds the size of the switch address table?

Some frames with unique addresses will be delivered in flood mode as if they have a broadcast address



1.        The user-defined filter includes a logical condition and the action that must be carried out over the frame if this condition is satisfied. Formulate the filter conditions that discard frames arriving from computer A, which has the 06 DB 00 34 5E 27 MAC address, and from computer B, which has the CC 33 00 D5 43 4D MAC address, to server S, whose MAC address is CC 33 00 65 44 AA.

Condition: ((Source_address = 06DB00345E27) OR (Source_address = CC3300D5434D) ) AND (Destination_address = CC33006544AA)

Action: Drop  


2.        It is necessary to improve the performance of a network based on a shared medium (Fig. 15.25). Only one switch is at your disposal; it has two 1,000 Mbps ports and eight 100 Mbps ports. How would you transform the network if it is possible to continue using the hubs available in the initial variant?

Fig. 15.25. Network to be upgraded

There are different versions of the switch that can result in a network performance improvement.  For example, it is possible to use two 1000 Mbps ports to connect to them the most powerful servers; the six 100 Mbps ports can be used to connect the most powerful workstations of different departments; the other two 100 Mbps ports can be connected to two existing hubs using one of them for connection of two workstations and the second – for connection three workstation provided that the workstations connected to the same hub belong to the same department.