Chapter 12


"Do I Know This Already?"

1.

b

2.

c

3.

a

4.

c

5.

a

6.

b

7.

c

8.

d

9.

c

10.

d

11.

b

12.

c

Q&A

1.

What is synchronization in RSTP?

Answer:

Because 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?

Answer:

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 fails.

3.

What is the difference between an alternate port and a backup port?

Answer:

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.

4.

Can a switch port be a designated port and be in the Discarding state?

Answer:

Yes. RSTP removes the link between a port's role and its state. In 802.1D, a designated port must be forwarding, but RSTP doesn't have the same requirement.

5.

Which port type cannot participate in RSTP synchronization?

Answer:

Edge port

6.

What two messages must be exchanged during RSTP synchronization?

Answer:

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.)

Answer:

The port is 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 from its CAM table?

Answer:

The switch flushes entries immediately instead of employing the timer reduction that 802.1D uses.

9.

What command configures a port as an RSTP edge port?

Answer:
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 can accomplish this?

Answer:
spanning-tree link-type point-to-point

11.

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

  1. MST

  2. PVST+

  3. CST

  4. 802.1D

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?

Answer:

The region name, configuration revision number, and 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?

Answer:

The VLAN-to-instance mapping is kept in a table of 4096 entries. This information is passed along in the MST BPDUs. Instead of 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?

Answer:

The Internal Spanning Tree (IST) instance

15.

Which MST instance is the IST?

Answer:

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+?

Answer:

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?

Answer:

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?

Answer:

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

19.

How can MST configuration information be propagated to other Cisco switches?

Answer:

The MST information must be configured manually in each switch within the MST region. Currently, no method or protocol automatically can propagate the MST configuration.

20.

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

Answer:

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.



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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