CCNA FAQ: Enhanced Switched Technologies

CCNA FAQ: Enhanced Switched Technologies

Q1. You receive the following output from a switch:
ccna-faq-enhanced-switched-technologies
Which are true regarding this switch? (Choose two.)
A. The switch is a root bridge.
B. The switch is a non-root bridge.
C. The root bridge is four switches away.
D. The switch is running 802.1w.
E. The switch is running STP PVST+.

Answer: B, D. The switch is not the root bridge for VLAN 1 or the output would tell us exactly that. We can see that the root bridge for VLAN 1 is off of interface G1/2 with a cost of 4, meaning it is directly connected. Use the command show cdp nei to fid your root bridge at this point. Also, the switch is running RSTP (802.1d), not STP.

Q2. If you want to effectively 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

Answer: 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. Understand this doesn’t completely disable STP, just stops any port confiured with PortFast from transitioning the different port states, and immediately puts the port into forwarding state.

Q3. Which of the following would you use to find the VLANs for which your switch is the root bridge? (Choose two.)
A. show spanning-tree
B. show root all
C. show spanning-tree port root VLAN
D. show spanning-tree summary

Answer: A, D. It is important that you can fid your root bridge, and the show spanning-tree command will help you do this. To quickly fid out which VLANs your switch is the root bridge for, use the show spanning-tree summary command.

Q4. 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

Answer: 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 because it has all the fies that the Cisco extensions provide with 802.1d. Remember, Cisco runs RSTP PVST+, not just RSTP.

Q5. Which of the following is a layer 2 protocol used to maintain a loop-free network?
A. VTP
B. STP
C. RIP
D. CDP

Answer: B. The Spanning Tree Protocol is used to stop switching loops in a layer 2 switched network with redundant paths.

Q6. 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.

Answer: 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.

Q7. Which of the following modes enable LACP EtherChannel? (Choose two.)
A. On
B. Prevent
C. Passive
D. Auto
E. Active
F. Desirable

Answer: C, E. There are two types of EtherChannel: Cisco’s PAgP and the IEEE’s LACP. They are basically the same, and there is little difference to confiuring them. For PAgP, use auto or desirable mode, and with LACP use passive or active. These modes decide which method you are using, and they must be confiured the same on both sides of the EtherChannel bundle.

Q8. Which of the following are true regarding RSTP? (Choose three.)
A. RSTP speeds the recalculation of the spanning tree when the layer 2 network topology changes.
B. RSTP is an IEEE standard that redefines STP port roles, states, and BPDUs.
C. RSTP is extremely proactive and very quick, and therefore it absolutely needs the 802.1 delay timers.
D. RSTP (802.1w) supersedes 802.1d while remaining proprietary.
E. All of the 802.1d terminology and most parameters have been changed.
F. 802.1w is capable of reverting to 802.1d to interoperate with traditional switches on a per-port basis.

Answer: A, B, F. RSTP helps with convergence issues that plague traditional STP. Rapid PVST+ is based on the 802.1w standard in the same way that PVST+ is based on 802.1d. The operation of Rapid PVST+ is simply a separate instance of 802.1w for each VLAN.

Q9. What does BPDU Guard perform?
A. Makes sure the port is receiving BPDUs from the correct upstream switch.
B. Makes sure the port is not receiving BPDUs from the upstream switch, only the root.
C. If a BPDU is received on a BPDU Guard port, PortFast is used to shut down the port.
D. Shuts down a port if a BPDU is seen on that port.

Answer: D. BPDU Guard is used when a port is confiured for PortFast, or it should be used, because if that port receives a BPDU from another switch, BPDU Guard will shut that rt down to stop a loop from occurring.

Q10. How many bits is the sys-id-ext field in a BPDU?
A. 4
B. 8
C. 12
D. 16

Answer: C. To allow for the PVST+ to operate, there’s a fild inserted into the BPDU to accommodate the extended system ID so that PVST+ can have a root bridge confiured on a per-STP instance. The extended system ID (VLAN ID) is a 12-bit fild, and we can even see what this fild is carrying via the show spanning-tree command output.

Q11. There are four connections between two switches running RSTP PVST+ and you want to figure out how to achieve higher bandwidth without sacrificing the resiliency that RSTP provides. What can you configure between these two switches to achieve higher bandwidth than the default configuration is already providing?
A. Set PortFast and BPDU Guard, which provides faster convergence.
B. Configure unequal cost load balancing with RSTP PVST+.
C. Place all four links into the same EtherChannel bundle.
D. Configure PPP and use multilink.

Answer: C. PortFast and BPDU Guard allow a port to transition to the forwarding state quickly, which is great for a switch port but not for load balancing. You can somewhat load balance with RSTP, but that is out of the scope of our objectives, and although you can use PPP to confiure multilink (bundle links), this is performed on asynchronous or synchronous serial links. Cisco’s EtherChannel can bundle up to eight ports between switches.

Q12. 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

Answer: 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; you will have broadcast storms and multiple frame copies being received by the same destination device..

Q13. You want to configure LACP. Which do you need to make sure are configured exactly the same on all switch interfaces you are using? (Choose three.)
A. Virtual MAC address
B. Port speeds
C. Duplex
D. PortFast enabled
E. VLAN information

Answer: B, C, E. All the ports on both sides of every link must be confiured exactly the same or it will not work. Speed, duplex, and allowed VLANs must match.

Q14. Which of the following modes enable PAgP EtherChannel? (Choose two.)
A. On
B. Prevent
C. Passive
D. Auto
E. Active
F. Desirable

Answer: D, F. There are two types of EtherChannel: Cisco’s PAgP and the IEEE’s LACP. They are basically the same, and there is little difference to confiure them. For PAgP, use the auto or desirable mode, and with LACP use the passive or active mode. These modes decide which method you are using, and they must be confiured the same on both sides of the EtherChannel bundle.

Q15. For this question, refer to the following illustration. SB’s RP to the root bridge has failed.
ccna-faq-enhanced-switched-technologies.jpg2
What is the new cost for SB to make a single path to the root bridge?
A. 4
B. 8
C. 23
D. 12

Answer: D. You can’t answer this question if you don’t know who the root bridge is. SC has a bridge priority of 4,096, so that is the root bridge. The cost for SB was 4, with the ect link, but that link went down. If SB goes through SA to SC, the cost would be 4 + 19, or 23. If SB goes to SA to SD to SC, the cost is 4 + 4 + 4 = 12.

Q16. Which of the following would put switch interfaces into EtherChannel port number 1, using LACP? (Choose two.)
A. Switch(config)#interface port-channel 1
B. Switch(config)#channel-group 1 mode active
C. Switch#interface port-channel 1
D. Switch(config-if)#channel-group 1 mode active

Answer: A, D. To confiure EtherChannel, create the port channel from global confiuration mode, and then assign the group number on each interface using the active mode to ble LACP, although just confiuring the channel-group command under your interfaces will enable the bundle, but options A and D are the best Cisco objective answers.

Q17. Which two commands would guarantee your switch to be the root bridge for VLAN 30? (Choose two.)
A. spanning-tree vlan 30 priority 0
B. spanning-tree vlan 30 priority 16384
C. spanning-tree vlan 30 root guarantee
D. spanning-tree vlan 30 root primary

Answer: A, D. You can set the priority to any value from 0 through 61,440 in increments of 4,096. Setting it to zero (0) means that the switch will always be a root as long as it has a lower MAC than another switch with its bridge ID also set to 0. You can also force a switch to be a root for a VLAN with the spanning-tree vlan vlan primary command.

Q18. Why does Cisco use its proprietary extension of PVST+ with STP and RSTP?

A. Root bridge placement enables faster convergence as well as optimal path determination.
B. Non-root bridge placement clearly enables faster convergence as well as optimal path determination.
C. PVST+ allows for faster discarding of non-IP frames.
D. PVST+ is actually an IEEE standard called 802.1w.

Answer: A. By using per-VLAN spanning tree, the root bridge can be placed in the center of where all the resources are for a particular VLAN, which enables optimal path rmination.

Q19. Which are states in 802.1d? (Choose all that apply.)
A. Blocking
B. Discarding
C. Listening
D. Learning
E. Forwarding
F. Alternate

Answer: A, C, D, E. Each 802.1d port transitions through blocking, listening, learning, and fially forwarding after 50 seconds, by default. RSTP uses discarding, learning, forwarding only.

Q20. Which of the following are roles in STP? (Choose all that apply.)
A. Blocking
B. Discarding
C. Root
D. Non-designated
E. Forwarding
F. Designated

Answer: A, C, D, E, F. The roles a switch port can play in STP are root, non-root, designated, non-designated, forwarding, and blocking. Discarding is used in RSTP, and disabled could be a role, but it’s not listed as a possible answer.

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