Review Questions

1.        What is the goal of STA?

An automatic building of a loop-free network topology to allow LAN switches use on a network with an arbitrary links topology. 

2.        Provide a definition of the spanning tree.

3.        Which port of the switch is called the root port?

The port that has a minimal distance to a root switch

4.        The designated port is the one defined as follows:

A.      The port that becomes the root port at discretion of the network administrator

B.       The port whose distance to the root switch has the minimum value for a specific segment - Correct

C.       The port switched to the blocked state

5.        How is the distance between switches measured in STA?

A distance is a value which is inversely proportional to a speed of a segment.  This value is measured in 10 nanosecond units. For instance, a 10 Mbps Ethernet a distance is 10 units.

6.        List the three stages of the process of building an active spanning tree topology.

7.        How is the root port chosen from several candidates for this role if their distances from the root switch are equal?

A port which has a minimal identifier.

8.        Is it possible for a network administrator to influence the choice of the root switch?

Yes, by means of an assigning a value to the two highest bytes of a switch identifier.

9.        How do switches decide that selection of the active topology has been accomplished?

By means of a time-out

10.     What triggers the switch to search for a new active topology?

An absence of a fresh BPDU from a root switch for a certain period of time (a time-out) 

11.     What is the main drawback of STA?

Relatively long duration of a spanning-tree topology building process

12.     Link aggregation:

A.      Improves network performance

B.       Improves network reliability

C.       Ensures both properties - Correct

13.     In what cases is it more efficient to employ link aggregation rather than to migrate to the faster version of the Ethernet technology?

When it is necessary to double or triple a speed of some link rather than tenfold it.

14.     How do STA and link aggregation interact?

STA treats an aggregated link as a single one.

15.     What are the limitations of the link aggregation techniques?

Most of the aggregation techniques allow aggregating only those links that connect the same neighbouring switches.  

16.     What is the difference between a unidirectional and a bidirectional trunk?

Each switch controls only process of frame sending, making a decision through which port the frame should be sent out. Therefore, if both neighbouring switches treat the connecting them parallel links as a trunk then the trunk will be bidirectional; otherwise it is unidirectional.


17.     What considerations are taken into account when choosing the trunk port for frame transmission?

Static frame distribution assumes that a specific port of the trunk is allocated for the frame flow of a specific session established between two nodes. All frames of that session will pass through the same queue, which ensures that they will arrive to the destination node in the same order as they were sent.

Dynamic frame distribution takes into account the current load of each port and ensures load balancing over all links of the trunk.


18.     Why is it necessary to take into account that frames belong to the same session when using aggregate links?

To preserve the order of transferring frames which increase a performance of some protocols?


19.     Why can VLAN be called a broadcast domain?

Because broadcast traffic generated by some node is not transmitted outside a VLAN which the node belongs to.


20.     How is it possible to join several VLANs?

There are two ways to do it:

-          To make some node a member of the VLANs which need to be connected;

-          To connect the VLANs by means of a router.


21.     List the main methods of creating VLANs.

-          Port grouping;

-          MAC-addresses grouping;

-          Using IEEE 802.1Q tags.


22.     Why is port grouping inefficient in networks based on several switches?

In this case as many ports of each switch should be dedicated to inter-switch connections as number of established VLANs


23.     Which approach to solving the problem of building VLANs on the basis of several switches is chosen in the IEEE 802.1Q standard?

To keep a VLAN tag in an additional field of a frame.


24.     Is it possible to use port grouping and the IEEE 802.1Q standard together?



25.     Is it necessary for STA to take into account VLAN presence within the network?


26.     Which QoS mechanisms are supported by LAN switches?

-          Traffic classification and marking;

-          Differentiated queuing on the basis of priority and weighted algorithms.


27.     What is the number of traffic classes recommended by the IEEE 802.1D-1998 standard?



28.     What should you do if network switches support a smaller number of queues than the number of traffic classes?

To map several classes onto the same queue.


29.     List the limitations of networks built on the basis of switches.