Summary
In this chapter, I talked about the differences between switches and bridges and how they both work at layer 2 and create a MAC address forward/filter table in order to make decisions on whether to forward or flood a frame.
I also discussed problems that can occur if you have multiple links between bridges (switches) and how to solve these problems by using the
Spanning Tree Protocol (STP).
Finally, I covered detailed configuration of Cisco’s Catalyst switches, including verifying the configuration, setting the Cisco STP extensions, and changing the root bridge by setting a bridge priority.
Exam Essentials
Remember the three switch functions. Address learning, forward/filter decisions, and loop avoidance are the functions of a switch.
Remember the command show mac address-table. The command show mac address-table will show you the forward/filter table used on the LAN switch. Understand the main purpose of the Spanning Tree Protocol in a switched LAN. The main purpose of STP is to prevent switching loops in a network with redundant switched paths.
Remember the states of STP. The purpose of the blocking state is to prevent the use of looped paths. A port in listening state prepares to forward data frames without populating the MAC address table. A port in learning state populates the MAC address table but doesn’t forward data frames. A port in forwarding state sends and receives all data frames on the bridged port. Last, a port in the disabled state is virtually nonoperational. Remember the command show spanning-tree. You must be familiar with the command show spanning-tree and how to determine who the root bridge is.
Written Lab 10
Write the answers to the following questions:
1. What command will show you the forward/filter table?
2. If a destination MAC address is not in the forward/filter table, what will the switch do with the frame?
3. What are the three switch functions at layer 2?
4. If a frame is received on a switch port and the source MAC address is not in the forward/filter table, what will the switch do?
5. Which proprietary Cisco STP extension would put a switch port into err-disabled if a BPDU is received on this port?
6. 802.1w is also called what?
7. When is STP considered to be converged?
8. Switches break up _________ domains.
9. What is used to prevent switching loops in a network with redundant switched paths?
10. Which Cisco 802.1d extension stops BPDU from being transmitted out a port?
(The answers to Written Lab 10 can be found following the review questions for this chapter.)
Review Questions
The following questions are designed to test your understanding of this chapter’s material. For more information on how to get additional questions, please see this book’s
Introduction.
1. Which of the following is a layer 2 protocol used to maintain a loop-free network?
A. VTP
B. STP
C. RIP
D. CDP
2. What command will display the forward/filter table?
A. show mac filter
B. show run
C. show mac address-table
D. show mac filter-table
3. What is the result of segmenting a network with a bridge (switch)? (Choose two.)
A. It increases the number of collision domains.
B. It decreases the number of collision domains.
C. It increases the number of broadcast domains.
D. It decreases the number of broadcast domains.
E. It makes smaller collision domains.
F. It makes larger collision domains.
4. Which statement describes a spanning-tree network that has converged?
A. All switch and bridge ports are in the forwarding state.
B. All switch and bridge ports are assigned as either root or designated ports.
C. All switch and bridge ports are in either the forwarding or blocking state.
D. All switch and bridge ports are either blocking or looping.
5. What is the purpose of Spanning Tree Protocol in a switched LAN?
A. To provide a mechanism for network monitoring in switched environments
B. To prevent routing loops in networks with redundant paths
C. To prevent switching loops in networks with redundant switched paths
D. To manage the VLAN database across multiple switches
E. To create collision domains
6. What are the three distinct functions of layer 2 switching that increase available bandwidth on the network? (Choose three.)
A. Address learning
B. Routing
C. Forwarding and filtering
D. Creating network loops
E. Loop avoidance
F. IP addressing
7. Your switch has a port status LED that is alternating between green and amber. What could this indicate?
A. The port is experiencing errors.
B. The port is shut down.
C. The port is in STP blocking mode.
D. Nothing; this is normal.
8. Which of the following statements is true?
A. A switch creates a single collision domain and a single broadcast domain. A router creates a single collision domain.
B. A switch creates separate collision domains but one broadcast domain. A router provides a separate broadcast domain.
C. A switch creates a single collision domain and separate broadcast domains. A router provides a separate broadcast domain as well.
D. A switch creates separate collision domains and separate broadcast domains. A router provides separate collision domains.
9. You need to configure a Catalyst switch so it can be managed remotely. Which of the following would you use to accomplish this task?
A. Switch(config)#int fa0/1
Switch(config-if)#ip address 192.168.10.252 255.255.255.0
Switch(config-if)#no shut
B. Switch(config)#int vlan 1
Switch(config-if)#ip address 192.168.10.252 255.255.255.0
Switch(config-if)#ip default-gateway 192.168.10.254 255.255.255.0
C. Switch(config)#ip default-gateway 192.168.10.254
Switch(config)#int vlan 1
Switch(config-if)#ip address 192.168.10.252 255.255.255.0
Switch(config-if)#no shut
D. Switch(config)#ip default-network 192.168.10.254
Switch(config)#int vlan 1
Switch(config-if)#ip address 192.168.10.252 255.255.255.0
Switch(config-if)#no shut
10. What does a switch do when a frame is received on an interface and the destination hardware address is unknown or not in the filter table?
A. Forwards the switch to the first available link
B. Drops the frame
C. Floods the network with the frame looking for the device
D. Sends back a message to the originating station asking for a name resolution
11. If a switch receives a frame and the source MAC address is not in the MAC address table but the destination address is, what will the switch do with the frame?
A. Discard it and send an error message back to the originating host
B. Flood the network with the frame
C. Add the source address and port to the MAC address table and forward the frame out the destination port
D. Add the destination to the MAC address table and then forward the frame
12. You want to run the new 802.1w on your switches. Which of the following would enable this protocol?
A. Switch(config)#spanning-tree mode rapid-pvst
B. Switch#spanning-tree mode rapid-pvst
C. Switch(config)#spanning-tree mode 802.1w
D. Switch#spanning-tree mode 802.1w
13. In which circumstance are multiple copies of the same unicast frame likely to be transmitted in a switched LAN?
A. During high-traffic periods
B. After broken links are reestablished
C. When upper-layer protocols require high reliability
D. In an improperly implemented redundant topology
14. Which command was used to produce the following output:
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 0005.dccb.d74b DYNAMIC Fa0/1
1 000a.f467.9e80 DYNAMIC Fa0/3
1 000a.f467.9e8b DYNAMIC Fa0/4
1 000a.f467.9e8c DYNAMIC Fa0/3
1 0010.7b7f.c2b0 DYNAMIC Fa0/3
1 0030.80dc.460b DYNAMIC Fa0/3
A. show vlan
B. show ip route
C. show mac address-table
D. show mac address-filter
15. If you want to disable STP on a port connected to a server, which command would you use?
A. disable spanning-tree
B. spanning-tree off
C. spanning-tree security
D. spanning-tree portfast
16. Refer to the diagram. Why does the switch have two MAC addresses assigned to the FastEthernet 0/1 port in the switch address table?

A. Data from HostC and HostD has been received by the switch port FastEthernet 0/1.
B. Data from two of the devices connected to the switch has been forwarded out to HostD.
C. HostC and HostD had their NIC replaced.
D. HostC and HostD are on different VLANs.
17. Layer 2 switching provides which of the following? (Choose four.)
A. Hardware-based bridging (ASIC)
B. Wire speed
C. Low latency
D. Low cost
E. Routing
F. WAN services
18. You type show mac address-table and receive the following output:
Switch#sh mac address-table
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 0005.dccb.d74b DYNAMIC Fa0/1
1 000a.f467.9e80 DYNAMIC Fa0/3
1 000a.f467.9e8b DYNAMIC Fa0/4
1 000a.f467.9e8c DYNAMIC Fa0/3
1 0010.7b7f.c2b0 DYNAMIC Fa0/3
1 0030.80dc.460b DYNAMIC Fa0/3
Suppose the above switch received a frame with the following MAC addresses:
Source MAC: 0005.dccb.d74b
Destination MAC: 000a.f467.9e8c
What will it do?
A. It will discard the frame.
B. It will forward the frame out port Fa0/3 only.
C. It will forward it out Fa0/1 only.
D. It will send it out all ports except Fa0/1.
19. You need to allow one host to be permitted to attach dynamically to each switch interface. Which two commands must you configure on your
Catalyst switch to meet this policy? (Choose two.)
A. Switch(config-if)#ip access-group 10
B. Switch(config-if)#switchport port-security maximum 1
C. Switch(config)#access-list 10 permit ip host 1
D. Switch(config-if)#switchport port-security violation shutdown
E. Switch(config)#mac-address-table secure
20. You have two switches connected together with two crossover cables for redundancy, and STP is disabled. Which of the following will happen between the switches?
A. The routing tables on the switches will not update.
B. The MAC forward/filter table will not update on the switch.
C. Broadcast storms will occur on the switched network.
D. The switches will automatically load-balance between the two links.
Answers to Review Questions
1. B. The Spanning Tree Protocol is used to stop switching loops in a switched network with redundant paths.
2. C. The command show mac address-table displays the forward/filter table on the switch.
3. A, E. Bridges break up collision domains, which would increase the number of collision domains in a network and also make smaller collision domains. 4. C. Convergence occurs when all ports on bridges and switches have transitioned to either the forwarding or blocking states. No data is forwarded until convergence is complete. Before data can be forwarded again, all devices must be updated.
5. C. The Spanning Tree Protocol (STP) was designed to stop layer 2 loops. All Cisco switches have the STP on by default.
6. A, C, E. Layer 2 features include address learning, forwarding and filtering of the network, and loop avoidance.
7. A. When you connect to a switch port, at first the link lights are orange/amber, and then they turn green, indicating normal operation. If the link light is blinking, you have a problem.
8. B. Switches break up collision domains, and routers break up broadcast domains.
9. C. To manage a switch remotely, you must set an IP address under the management VLAN, which is, by default, interface vlan 1. Then, from global configuration mode, you set the default gateway with the ip default-gateway command. Option C enables the management interface, which makes it correct over option B.
10. C. Switches flood all frames that have an unknown destination address. If a device answers the frame, the switch will update the MAC address table to reflect the location of the device.
11. C. Since the source MAC address is not in the MAC address table, the switch will add the source address and the port it is connected to into the MAC address table and then forward the frame to the outgoing port.
12. A. 802.1w is the also called Rapid Spanning Tree Protocol. It is not enabled by default on Cisco switches, but it is a better STP to run since it has all the fixes that the Cisco extensions provide with 802.1d.
13. D. If the Spanning Tree Protocol is not running on your switches and you connect them together with redundant links, you will have broadcast storms and multiple frame copies.
14. C. The command show mac address-table will display the forward/filter table, also called a CAM table, on a switch.
15. D. If you have a server or other devices connected into your switch that you’re totally sure won’t create a switching loop if STP is disabled, you can use something called portfast on these ports. Using it means the port won’t spend the usual 50 seconds to come up while STP is converging.
16. A. A switch can have multiple MAC addresses associated with a port. In the diagram, a hub is connected to port Fa0/1, which has two hosts connected. 17. A, B, C, D. Switches, unlike bridges, are hardware based. Cisco says its switches are wire speed and provide low latency, and I guess they are low cost compared to their prices in the 1990s.
18. B. Since the destination MAC address is in the MAC address table (forward/filter table), it will send it out port Fa0/3 only.
19. B, D. switchport port-security is an important command, and it’s super easy with the CNA; however, from the CLI you can set the maximum number of MAC addresses allowed into the port and then set the penalty if this maximum has been passed.
20. C. If spanning tree is disabled on a switch and you have redundant links to another switch, broadcast storms will occur, among other possible problems. Answers to Written Lab 10
1. show mac address-table
2. Flood the frame out all ports except the port on which it was received
3. Address learning, forward/filter decisions, and loop avoidance
4. It will add the source MAC address in the forward/filter table and associate it with the port on which the frame was received.
5. BPDUGuard
6. Rapid Spanning Tree Protocol (RSTP)
7. When all ports are in either the blocking or the forwarding mode
8. Collision
9. Spanning Tree Protocol (STP)
10. PortFast