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CCNP 642-811 BCMSN Exam Certification Guide - Cisco press

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556Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

2.What condition must be met to keep a switch port in the blocking state?

A constant flow of BPDUs. Without them, the switch thinks there is no need to block the port any longer.

3.If a switch port is shown to be in the root-inconsistent state, what has happened on it?

Root guard has detected someone advertising a BPDU that is superior to the current Root Bridge.

4.When root guard has been triggered on a switch port, what must be done to enable the port for use again?

Root guard will automatically allow the port to be moved through the STP states as soon as the superior BPDUs are no longer received.

5.When BPDU guard is enabled on a switch port, what state will the port be put in if a BPDU is received on it?


6.When BPDU guard has been triggered on a switch port, what must be done to enable the port for use again?

If the errdisable timeout feature has been enabled, the switch will automatically put the switch port back into service after a set amount of time. Otherwise, the port will remain disabled until you manually enable it again.

7.What can happen if BPDUs are delayed or “skewed” as they pass along from switch to switch?

A blocking link can be moved to forwarding, creating a bridging loop, if BPDUs are delayed or dropped.

8.What action does BPDU skew detection take when late BPDUs are detected?

None, other than generating a syslog message to alert someone of the skewed condition.

9.When loop guard is enabled on a switch port, what state will the port be put in if BPDUs are noted to be missing?


Q&A 557

10.Can STP loop guard be enabled on all switch ports?

Yes; only the non-designated ports will be affected by loop guard.

11.When UDLD is enabled on a switch port, what else must be done to detect a unidirectional link on the port?

Enable UDLD on the far-end switch on the same link.

12.What is the difference between the UDLD normal and aggressive modes?

Normal mode detects a unidirectional link condition and reports it. Aggressive mode detects the condition, reports it, and moves the port to the errdisable state so that it can’t be used.

13.What command enables UDLD aggressive mode on a switch interface? udld aggressive

14.If two switches enable UDLD on the ports that have a common link, do their UDLD message times have to agree?

No. The UDLD messages are just echoed back, so the message times are only locally significant.

15.UDLD should be used on switch ports with what type of media? Fiber-optic media

16.Can UDLD be used on all switch ports without causing problems?

Yes, although it is needed only on all fiber-based ports. The device on the far end of the port connection must also support UDLD so that UDLD messages can be echoed back to the switch.

17.Complete the following command to display all ports that are disabled due to STP protection features: show spanning-tree

show spanning-tree inconsistentports

558 Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

Chapter 12

“Do I Know This Already?”














1.What is synchronization in RSTP?

As RSTP works to converge a switched network, each switch effectively isolates itself from the next layer of neighbors until an agreement can be reached about who will have the designated port on each segment.

2.What is an Alternate Port?

A port with an alternate path to the Root. The path is less desirable than the one through the Root Port but is flagged for immediate use if the Root Port path should fail.

3.What is the difference between an Alternate Port and a Backup Port?

An Alternate Port connects to a different segment than the Root Port to provide an alternate path to the Root. A Backup Port connects to the same segment as another port on the local switch to provide another path out of the switch, but not necessarily another path back to the Root.

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4.Can a switch port be a Designated Port and be in the Discarding state?

Yes; RSTP removes the linkage between a port’s role and its state. In 802.1D, a Designated Port must be forwarding, but RSTP doesn’t make the same requirement.

5.Which of the following ports can participate in RSTP synchronization?

a.Root Port

b.Designated Port

c.Edge Port

d.Nonedge Port

e.Point-to-point Port

Answer: a, b, d, e

6.What two messages must be exchanged during RSTP synchronization? Proposal and Agreement

7.After an agreement message is received from a neighboring switch, how much time elapses before the port can begin forwarding? (Consider any timers that must expire or other conditions that must be met.)

The port will be moved to the Forwarding state immediately after the agreement message is received. With RSTP, no other conditions are necessary because two switches have completed a quick handshake by exchanging proposal and agreement messages.

8.After a switch receives news of a topology change, how long does it wait to flush entries out of its CAM table?

The switch flushes entries immediately, rather than employing the timer reduction that 802.1D uses.

9.What command configures a port as an RSTP edge port? spanning-tree portfast

10.Suppose interface FastEthernet 0/1 is in half-duplex mode, but you want it to be considered a point-to-point link for RSTP. What command will accomplish this?

spanning-tree link-type point-to-point

560Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

11.Put the following in order of the number of supported STP instances, from lowest to highest:





Answer; d, c, a, b (d and c both have a single instance.)

12.What three parameters must be configured to uniquely define an MST region?

The region name, configuration revision number, and the instance-to-VLAN mappings

13.What parameter does a switch examine to see if its neighbors have the same VLAN to MST instance mappings? How is that information passed among switches?

The VLAN-to-instance mapping is kept in a table of 4096 entries. This information is passed along in the MST BPDUs. Rather than passing the entire table, switches include only a digest of their current table contents.

14.Which MST instance in a region corresponds to the CST of 802.1Q? The Internal Spanning Tree (IST) instance

15.Which MST instance is the IST?

IST is Instance 0. Instances 1 through 15 are available for other use.

16.When an MST region meets a PVST+ domain, how is each MST instance propagated into PVST+?

The BPDUs from each instance are replicated and sent into all the appropriate VLANs in the PVST+ switch.

17.Is it wise to assign VLANs to MST Instance 0? Why or why not?

No. By default, all VLANs are mapped to Instance 0, the IST. You should select the number of instances needed and map all active VLANs to them. Otherwise, you can’t have full control over the topologies independent of IST and CST.

18.The commands have just been entered to define an MST region on a switch. You are still at the MST configuration prompt. What command must you enter to commit the MST changes on the switch?

exit (When the MST configuration mode is exited, the changes are committed immediately.)

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19.Which of the following methods can you use to assign or propagate MST configuration information to other switches?

a.Manual configuration




Answer: a

20.A switch can interact with both 802.1D and RSTP. Can it run both PVST+ and MST simultaneously?

No. A switch can run either PVST+ or MST. If a switch is running MST, it can interact and interoperate with PVST+, 802.1D, and RSTP.

Chapter 13

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562 Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections


1.What might you need to implement interVLAN routing? One or more Layer 3 interfaces

One or more SVIs Static routes

A dynamic routing protocol

2.Can interVLAN routing be performed over a single trunk link?

Yes; packets can be forwarded between the VLANs carried over the trunk.

3.To configure an SVI, what commands are needed? (First, make sure the VLAN is defined on the switch.) interface vlan vlan-id

ip address ip-address mask no shutdown

4.What command can verify the VLAN assignments on a Layer 2 port? show interface type mod/num switchport


show interface status

5.A switch has the following interface configurations in its running configuration:

interface fastethernet 0/1 switchport access vlan 5 interface vlan 5

ip address no shutdown

What is necessary for packets to get from the FastEthernet interface to the VLAN 5 SVI?

Nothing; both are assigned to VLAN 5, so normal Layer 2 transparent bridging will take care of all forwarding between the two.

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6.What is the source of FIB information?

The routing table, as computed by the Layer 3 engine portion of a switch.

7.How often is the FIB updated?

As needed; it is downloaded or updated dynamically by the Layer 3 engine whenever the routing topology changes or an ARP entry changes.

8.What is meant by the term “CEF punt?”

A packet can’t be forwarded or switched by CEF directly because it needs further processing. The packet is “punted” to the Layer 3 engine, effectively bypassing CEF for a more involved resolution.

9.What happens to the FIB when distributed CEF (dCEF) is used?

It is simply replicated to each of the independent CEF engines. The FIB itself remains intact, so that each engine receives a duplicate copy.

10.What happens during a “CEF glean” process?

The MAC address (ARP reply) for a next-hop FIB entry is not yet known. The Layer 3 engine must generate an ARP request and wait for a reply before CEF forwarding can continue to that destination.

11.What does a multilayer switch do to the IP TTL value just before a packet is forwarded? The TTL is decremented by one, as if a router had forwarded the packet.

12.What is fallback bridging?

On switch platforms that cannot multilayer switch (route) all routable protocols, those protocols can be transparently bridged between VLANs instead.

13.Is it possible for an SVI to go down? If so, what are the reasons?

Yes; the SVI can be administratively shut down with the shutdown command, as with any other interface. Also, if the VLAN associated with the SVI is not defined or active, the SVI will appear to be down.

564 Appendix A: Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections

Chapter 14

“Do I Know This Already?”














1.A multilayer switch has been configured with the command standby 5 priority 120. What router redundancy protocol is being used?


2.What feature can you use to prevent other routers from accidentally participating in an HSRP group?

HSRP authentication

3.What command can configure an HSRP group to use a virtual router address of

standby 1 ip

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4.The show standby vlan 271 command produces the following output:

Vlan271 - Group 1

Local state is Active, priority 210, may preempt

Hellotime 3 holdtime 40 configured hellotime 3 sec holdtime 40 sec Next hello sent in 00:00:00.594

Virtual IP address is configured Secondary virtual IP address Secondary virtual IP address

Active router is local

Standby router is unknown expires in 00:00:37 Standby virtual mac address is 0000.0c07.ac01 2 state changes, last state change 5d17h

If the local router fails, which router will take over the active role for the virtual router address

None. There is no known standby router, so it has also failed.

5.What is meant by preempting in HSRP?

Normally, if the active router (highest priority) fails, another router takes over its active role. The original active router is not allowed to resume the active role when it is restored until the new active router fails. Pre-empting allows a higher-priority router to take over the active role immediately or after a configurable delay.

6.What protocols discussed in this chapter support interface tracking? HSRP


7.The show standby brief command has been used to check the status of all HSRP groups on the local router. The output from this command is as follows:

Switch# show standby brief







P indicates configured to preempt.










Grp Prio P State

Active addr

Standby addr

Group addr




P Active





P Active





P Active


Each interface is shown to have Group 1. Is this a problem?

No, as long as there is no bridging between VLANs. The HSRP group number can be repeated because each group is isolated on its own VLAN.

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