Answers to Chapter 5 QA Section

1 What is EtherChannel? What types of switch links can it be used with?

EtherChannel is a method for aggregating multiple physical Ethernet ports into a single logical link. EtherChannel can be used with full-duplex Fast Ethernet or Gigabit Ethernet links.

2 How does an EtherChannel distribute broadcasts and multicasts?

Broadcasts and multicasts are sent across only one port of the bundle and are not distributed across the EtherChannel.

3 How is traffic distributed over an EtherChannel?

Traffic is distributed according to addresses contained in frames passing through the switch—not according to port loads or equal distribution across the individual ports in a bundle. Switches use an XOR computation of source, destination, or both addresses of either MAC or IP, depending on the switch capabilities.

4 What CLI-based switch command could be used to configure a 4-port EtherChannel on switch ports 3/1, 3/2, 3/3, and 3/4? The switch should use PAgP to actively negotiate the EtherChannel.

set port channel 3/1-4 mode desirable

5 What is PAgP used for?

PAgP is a protocol that is used to dynamically and automatically configure an EtherChannel over multiple physical ports.

6 What happens if one port of an EtherChannel is unplugged or goes dead? What happens when that port is reconnected?

Traffic on the disconnected port will be moved to the next available link in the EtherChannel bundle. When the port is reconnected, traffic will not automatically move back to the original port of the bundle. Rather, new traffic will be learned and applied to the restored link.

7 What is a bridging loop? Why is it bad?

A bridging loop is a path through a bridged or switched network that provides connectivity in a loop. Broadcast or multicast frames introduced into the loop are propagated by each switch causing the frames to circulate around and around the loop. Network bandwidth and CPU resources can be completely absorbed by the increasing amount of broadcast traffic. Bridging loops can ended by breaking the loop connectivity.

8 Put the following STP port states in chronological order:

a. Learning b. Forwarding c. Listening d. Blocking d. Blocking c. Listening a. Learning b. Forwarding

9 Name two types of Spanning-Tree Protocol messages used to communicate between bridges.

Configuration BPDUs and Topology Change Notification BPDUs.

10 What commands can be used to configure a CLI-based switch as the Root Bridge on VLAN 10, assuming that the other switches are using the default STP values?

The following command can be used to directly set the Bridge Priority value:

set spantree priority 100 10

Any priority value less than the default 32768 will promote the switch to Root Bridge status. A value of 100 is easy to use and remember.

Another method is to use the following command, which directly sets the switch to Root Bridge and offers an easy user interface with no values to remember or compare:

set spantree root 10

11 Using your Root Bridge answer from question 10, what commands can be used to configure another CLI-based switch as a secondary or backup Root Bridge on VLAN 10?

Using the example answer for question 10, the Root Bridge has been given a priority of 100. A secondary Root Bridge can be configured by using a Bridge Priority value that is greater than 100 and less than the default 32768. In this example answer, a secondary priority value of 200 is used, because it too is easy to remember:

set spantree priority 200 10

Another method is to directly configure the switch as a secondary Root Bridge by using this command:

set spantree root secondary 10

12 What criteria are used to select the following:

a. Root Bridge b. Root Port c. Designated Port d. Redundant (or secondary) Root Bridges a. Lowest Bridge ID (Bridge priority, MAC address)

b. Lowest Root Path Cost c. Lowest Root Path Cost on a shared segment d. Next-to-lowest Bridge ID

If a tie occurs, then these parameters are used to decide:

1. Lowest Bridge ID

2. Lowest Root Path Cost

3. Lowest Sender Bridge ID

4. Lowest Port ID

13 Which of the following switches will become the Root Bridge, given the information in the table below? Which switch will become the secondary Root Bridge, in the event that the Root Bridge fails?

Switch Name

Bridge Priority

MAC Address

Port Costs

Catalyst A



All are 19

Catalyst B



All are 4

Catalyst C



All are 19

Catalyst D



All are 19

The Root Bridge will be Catalyst C, because its Bridge Priority has the lowest value. The Bridge Priority is more significant because it is stored in the upper bits of the Bridge ID field. If Catalyst C fails in its duty as Root Bridge, then Catalyst B will take over as the secondary Root Bridge. Because Catalyst B has the default Bridge Priority (32768), along with another switch, the lowest MAC address will be the deciding factor.

14 What conditions cause a STP topology change? What effect does this have on STP and the network?

A topology change occurs when a port moves to the Forwarding state or from Forwarding or Learning to the Blocking state. During a topology change, addresses are aged out in Forward Delay seconds, while active stations are not aged out of the bridging table. The STP is not recomputed; TCN BPDUs are sent throughout the network notifying other switches of the topology change. Only the port where the topology change is occurring is affected by moving through the STP states.

15 A Root Bridge has been elected in a switched network. Suppose a new switch is installed with a lower Bridge ID than the existing Root Bridge. What will happen?

After the new switch comes up, a Root Bridge election will take place. This will occur at the next Hello time when the new switch announces itself as root. It will become the Root Bridge because it has the lowest Bridge ID, and the Spanning Tree topology will be recomputed. Where switch ports change state as a result of the election and topology change, outages will occur until the Forwarding state starts again.

16 What is the single-most important design decision to be made in a network running STP? Root Bridge placement.

17 Where should the Root Bridge be located in a switched network?

The Root Bridge should be placed as close to the center of the network as possible. For example, in a hierarchical design, the Root Bridge should be located in the Distribution layer.

18 Suppose a switch receives Configuration BPDUs on two of its ports. Both ports are assigned to the same VLAN. Each of the BPDUs announces Catalyst A as the Root Bridge. Can the switch use both of these ports as Root Ports? Why?

No. By definition, only one Root Port is selected on each switch. The port with the lowest Root Path Cost will be chosen.

19 What happens to a port that is neither a Root Port nor a Designated Port? That port is placed in the Blocking state so that no bridging loops form from it.

20 Suppose you need to troubleshoot your Spanning Tree topology and operation. What commands and information can you use on a switch to find information about the current STP topology?

Because it won't be obvious which switch in your network is the Root Bridge, you can begin on any switch with the show spantree vlan command. This command will show the current Root Bridge ID, Root Port, Designated Port, costs, timers, and port states. Consider the following example:

Switch> show spantree 11


Spanning tree enabled

Designated Root Designated Root Priority Designated Root Cost Designated Root Port Root Max Age 20 sec

Hello Time 2 sec Forward Delay 15 sec

Bridge ID MAC ADDR Bridge ID Priority Bridge Max Age 20 sec

00-50-bd-19-6c-00 32768

Hello Time 2 sec Forward Delay 15 sec

Port,Vlan Vlan Port-State Cost Priority Fast-Start Group-method

1/1 11 not-connected 1/2 11 forwarding Switch>

32 disabled 32 disabled

From this information, you can discover the MAC address of the Root Bridge. Sometimes it can be helpful to "walk" the Spanning Tree topology to find the Root Bridge. To do this, you would need to Telnet to the next upstream switch to get one hop closer to the Root. Find the Root Port, listed above. Then use the show cdp neighbor [module/port] detail command to find the IP address of the upstream neighbor. Using this methodology, you can trace every hop to the Root Bridge.

21 How is the Root Path Cost calculated for a switch port?

The Root Path cost is a cumulative value that is incremented as Configuration BPDUs are passed from switch to switch. A switch adds the Port Cost of its local port to the current Root Path Cost value as a BPDU is received.

22 What conditions can cause ports on the Root Bridge of a network to move into the Blocking state? (Assume that all switch connections are to other switches. No crossover cables are used to connect two ports together on the same switch.)

None. Every active port on the Root Bridge becomes a Root Port, since the Root Path Cost is zero. By this definition, the ports can never be in the Blocking state.

23 What is the maximum number of Root Ports that a Catalyst switch can have?

Only one. STP works to find a single path from each switch back to the Root Bridge.

24 What mechanism is used to set the STP timer values for all switches in a network?

The timers are set on the Root Bridge, and the values are propagated to all other switches by including them in Configuration BPDUs.

25 What parameters can be tuned to influence the selection of a port as a Root or Designated Port?

Port Cost and Port Priority.

26 What CLI-based command can be used to enable fast STP convergence for a single workstation on switch port 3/7?

set spantree portfast 3/7 enable

27 What technology can be useful to decrease the amount of time STP keeps an end user's workstation in the Blocking state when it powers up?

PortFast, which moves the port immediately into the Forwarding state.

28 What happens if the STP Hello Time is decreased to one second in an effort to speed up STP convergence? What happens if the Hello Time is increased to ten seconds?

Setting the Hello Timer to one second doubles the amount of Configuration BPDUs that are sent by a switch, as compared to the default 2 second timer. While this does share BPDU information more often, it really doesn't help the long convergence delay when a port comes up. The significant delays come from the Forward Delay timer, which is used to move a port through the Listening and Learning states. By default, this process takes 30 seconds and is unaffected by the Hello Timer.

29 Where should the UplinkFast feature be used in a switched network?

Only on switches that are leaf-nodes in the Spanning Tree topology, such as the Access layer.

30 What CLI-based switch command can be used to safely adjust the STP timers on the Root Bridge in VLAN 7? Assume that the network consists of Catalyst A, B, and C, all connected to each other in a triangle fashion.

Because the three switches form a triangle loop, one link will eventually be placed in the Blocking state. Therefore, the maximum distance across the network is 3 switch hops. This value can be used to define the network diameter to safely adjust the STP timers for faster convergence:

set spantree root dia 3

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