Chapter 3: Advanced STP Tuning

Question 1

Ideal root bridge placement is located on a _______ switch?

A. Core
B. Distribution
C. Access

Answer 1

Core

Question 2

What ensures that a switch is chosen as the root bridge?

Answer 2

Lowering the priority to 0.

Question 3

What command is used to modify spanning tree priority?

Answer 3

spanning-tree vlan [vlan id] priority [#]

Question 4

What command executes a script that modifies STP priority?

Answer 4

spanning-tree vlan [#] root {primary|secondary}

Question 5

What is the spanning-tree vlan priority range?

Answer 5

0-61440 in increments of 4096

Question 6

spanning-tree vlan [#] root primary modifies the vlan priority to what vlaue?

Answer 6

24576

Question 7

spanning-tree vlan [#] root secondary modifies the vlan priority to what vlaue?

Answer 7

28672

Question 8

What command is used to change STP port cost for a specific vlan?

Answer 8

spanning tree [vlan #] cost [#]

Question 9

Places a port in the ErrDisabled state if a superior BPDU is received on a configured port.

Answer 9

Root Guard.

Question 10

What STP feature is used to disable TCN generation on access ports?

Answer 10

Portfast

Question 11

Causes a port to begin forwarding traffic immediately.

Answer 11

Portfast

Question 12

What happens if a BPDU is received on a portfast-enabled port?

Answer 12

Portfast is removed from the port.

Question 13

True or False:

Portfast can only be enabled in interface configuration mode.

Answer 13

False.

Portfast can be enabled globally or on a port-by-port basis.

Question 14

In what scenario can a trunk port have portfast enabled?

Answer 14

When the port is connected to a single host.

ex. server with one NIC running a hypervisor with VMs on different VLANs.

Question 15

What is the command to enable portfast on all access ports?

Answer 15

spanning-tree portfast default

Question 16

What STP feature shuts down portfast-enabled ports if a BPDU is received?

Answer 16

BPDU guard

Question 17

By default, does a port that is put in the ErrDisable state by BPDU guard automatically restore itself?

Answer 17

No.

Question 18

What command is used to automatically restore a port placed in the ErrDisable state by BPDU guard?

Answer 18

errdisable recovery cause bpduguard

Question 19

Default Error Recovery service time

Answer 19

300 seconds

Question 20

This STP feature blocks BPDUs from being sent/received on a port.

Answer 20

BPDU filter

Question 21

BPDU guard global config command.

BPDU guard interface config command.

Answer 21

BPDU guard global config command
#spanning-tree portfast bpduguard default

BPDU guard interface config command.
#spanning-tree bpduguard {enable | disable}

Question 22

Prevents alternative or root ports from becoming designated ports due to loss of BPDUs on the root port.

Answer 22

Loop guard

Question 23

Places a port in ErrDisabled state while BPDUs are not being received.

Answer 23

Loop guard

Question 24

What mode is loop guard configured?

Answer 24

global config

Question 25

Monitors bidirectional traffic on fiber optic cables

Answer 25

UDLD

Question 26

UDLD operation

Answer 26

1. UDLD packets containing system id and port id are sent to neighboring switch.
2. Neighbor repeats back the information and sends its own system id and port id.

Question 27

Describe the 2 UDLD modes.

Answer 27

Normal: Unacknowledged frames cause the link to be considered undetermined and port remains inactive.

Aggressive: When a frame is not acknowledged, the switch sends another eight packets in 1-second intervals. Port is placed into error state if packets are not acknowledged.

Question 28

What command enables UDLD globally?

Answer 28

udld enable [aggressive]

This enables udld on any SFP port.

Question 29

Display the list of STP ports in an inconsistent state.

Answer 29

show spanning-tree inconsistentports