Chapter 11


"Do I Know This Already?"

1.

b

2.

c

3.

c

4.

b

5.

b

6.

a

7.

b

8.

b

9.

c

10.

c

11.

c

12.

b

Q&A

1.

Why would a unidirectional link be bad?

Answer:

Switches must exchange BPDUs in both directions across a link. If one side of the link is disrupted and the switches think the link is still operational, one of the switches will not receive BPDUs. If that switch had its end of the link in the Blocking state to prevent a bridging loop, the absence of BPDUs will cause it to promote the link toward the Forwarding state. At that point, the loop will form.

2.

What condition must be met to keep a switch port in the Blocking state?

Answer:

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?

Answer:

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?

Answer:

Root guard automatically allows the port to be moved through the STP states as soon as the superior BPDUs no longer are 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?

Answer:

errdisable

6.

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

Answer:

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

7.

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

Answer:

loop-inconsistent

8.

Can STP loop guard be enabled on all switch ports?

Answer:

Yes. Only the nondesignated ports are affected by loop guard.

9.

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

Answer:

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

10.

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

Answer:

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

11.

What command enables UDLD aggressive mode on a switch interface?

Answer:

udld aggressive

12.

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

Answer:

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

13.

UDLD should be used on switch ports with what type of media?

Answer:

Fiber-optic media

14.

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

Answer:

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

15.

Is it possible to disable STP on a single switch port without disabling the whole STP instance? If so, how can you do that?

Answer:

Yes, you can use BPDU filtering to effectively disable STP on a switch port. BPDUs are neither processed when they arrive nor sent on the switch port. You can use the spanning-tree bpdufilter enable interface-configuration command to enable BPDU filtering. Otherwise, the whole STP instance for a VLAN must be disabled.

16.

Complete the following command to display all ports that are disabled because of STP protection features:

show spanning-tree ______________

Answer:
show spanning-tree inconsistent ports



CCNP Self-Study(c) CCNP BCMSN Exam Certification Guide
Red Hat Fedora 5 Unleashed
ISBN: N/A
EAN: 2147483647
Year: 2003
Pages: 177

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