UDLD

Question 1

What does UDLD stand for?

Answer 1

Uni-Directional Link Detection.

Question 2

What does UDLD do?

Answer 2

protects STP when a physical malfunction only allows traffic in 1 direction, even though the link shows as up (cisco proprietary)

Question 3

How does UDLD work?

Answer 3

sends special layer 2 UDLD frames and expects an echo. Both ends must be configured for UDLD

Question 4

How should UDLD be configured?

Answer 4

the configureable UDLD interval must be less than max age plus two intervals of forward delay

Question 5

What are the 2 UDLD modes?

Answer 5

normal and aggressive

Question 6

What is UDLD normal mode?

Answer 6

if a unidirectional link is detected, the port continues normally, but the port is marked as undetermined and a syslog is generated

Question 7

What is UDLD aggressive mode?

Answer 7

If a unidirectional link is detected, the switch doesn’t try to reestablish the link. ULD msgs are sent once/sec for 8 seconds, then the port is err-disabled

Question 8

How is UDLD configured?

Answer 8

per-port or globally for all fiber-optic ports. Can be enabled globally, but will only affect fiber ports

Question 9

Define FLPs.

Answer 9

Fast link pulses. When using auto negotiation, these determine speed and duplex.

Question 10

Which statement about the UDLD protocol is true?

Answer 10

UDLD is a Cisco-proprietary Layer 2 protocol that enables devices to monitor the physical status of links and detect unidirectional failures

Question 11

Which option lists the modes that are available for configuring UDLD on a Cisco switch?

Answer 11

normal and aggressive

Question 12

While working in the core network building, a technician accidently bumps the fiber connection between two core switches and damages one of the pairs of fiber. As designed, the link was placed into a non-forwarding state due to a fault with UDLD. After the damaged cable was replaced, the link did not recover. What solution allows the network switch to automatically recover from such an issue?

Answer 12

errdisable autorecovery

Question 13

After UDLD is implemented, a Network Administrator noticed that one port stops receiving UDLD packets. This port continues to reestablish until after eight failed retries. The port then transitions into the errdisable state. Which option describes what causes the port to go into the errdisable state?

Answer 13

UDLD port is configured in aggressive mode.

Question 14

After reviewing UDLD status on switch ports, an engineer notices that the switch LEDs are green. Which statement describes what this indicates about the status of the port?

Answer 14

The port is fully operational and no known issues are detected.

Question 15

where is UDLD enabled?

Answer 15

all fiber-optic links between switches (must be enabled on both ends)

Question 16

Can loop guard and udld be used together?

Answer 16

yes

Question 17

can root guard and udld be used together?

Answer 17

yes

Question 18

What is UDLD?

Answer 18

UDLD is a Cisco proprietary Layer-2 feature. It is able to detect problems on a link without relying on BPDUs. The way this is done is by sending CDP-like messages that MUST be echoed back (with few alterations)

Question 19

What type of links is UDLD designed to be used on?

Answer 19

UDLD was originally designed for Fiber-Optic Links. Regular copper normally does not encounter UDLD issues.

Question 20

What are the modes of UDLD?

Answer 20

UDLD has two modes - Normal and Aggressive. Normal just prints out a syslog message, while aggressive pushes violations into error-disabled mode after 3 misses.

Question 21

What are some useful show commands when verifying UDLD?

Answer 21

Switch# show udld neighbors
Switch# show udld <interface></interface>

Question 22

How does UDLD work to protect STP?

Answer 22

It sends a special UDLD frame to the other end of a link, and that link replies to it to ensure that a link is truly bi-directional.

Question 23

How does UDLD work on EtherChannel ports?

Answer 23

It exchanges and echos messages on each link independently.

Question 24

What are the 2 modes of UDLD operation?

Answer 24

Normal and Aggressive.-Normal: When a unidirectional link is detected, it is still allowed to continue its operation, it marks the port as having an undetermined state, and generates a syslog message. -Aggressive: The port is placed in the errDisabled state after 8 UDLD messages sent one second apart.

Question 25

Where must you configure UDLD for it to work?

Answer 25

on both ends of the link.

Question 26

What is a Cisco proprietary method to detect or prevent the unexpected loss of BPDUs?

Answer 26

Unidirectional Link Detection (UDLD).

Question 27

Define UDLD

Answer 27

A protection against problems caused by unidirectional links between two switches. Uses messaging between switches to detect the loop, err-disabling the port when the link is unidirectional.