| UDLD quick overview: + Detect the unidirectional links (one-way communication) and helps preventing Layer 2 loops + Cisco proprietary + Layer 2 protocol (but works with Layer 1 mechanisms) + UDLD is disabled by default on all ports + Switch usually sends UDLD message (with the UDLD well-known MAC address 01:00:0C:CC:CC:CC) to the far end device every 15 seconds and expects UDLD messages to be returned. If it is not returned, we may have a unidirectional link + Two modes: – Normal mode: If UDLD detects a unidirectional link, it just marks this port as “Undetermined”, but does NOT shut down or disable the port. A syslog message is also generated. – Aggressive mode (recommended mode): If UDLD detects a unidirectional link, it is going to send a UDLD message every one second for eight seconds. If UDLD does not see any of these messages returned to itself, it would put that port into error-disabled state. + Can be used along with Etherchannel. If UDLD detects unidirectional link in one of the physical connections in the Etherchannel bundle, UDLD only puts that faulty physical link into error-disabled state (not the whole Etherchannel bundle) + Can be enabled globally or per-port basis.Note: A unidirectional link occurs whenever the traffic transmitted by a local device over a link is received by a neighbor, but traffic transmitted from the neighbor is not received by the local device |
Question 1Which statement about the UDLD protocol is true?
A. UDLD is a Cisco-proprietary Layer 2 protocol that enables devices to monitor the physical status of links and detect unidirectional failures.
B. UDLD is a Cisco-proprietary Layer 2 protocol that enables devices to advertise their identity, capabilities, and neighbors on a local area network.
C. UDLD is a standardized Layer 2 protocol that enables devices to monitor the physical status of links and detect unidirectional failures.
D. UDLD is a standardized Layer 2 protocol that enables devices to advertise their identity, capabilities, and neighbors on a local area network.
Answer: A
Explanation
UDLD is a Layer 2 protocol that enables devices connected through fiber-optic or twisted-pair Ethernet cables to monitor the physical configuration of the cables and detect when a unidirectional link exists. All connected devices must support UDLD for the protocol to successfully identify and disable unidirectional links. When UDLD detects a unidirectional link, it administratively shuts down the affected port and alerts you. Unidirectional links can cause a variety of problems, including spanning-tree topology loops.
Question 2Which option lists the modes that are available for configuring UDLD on a Cisco switch?
A. normal and aggressive
B. active and aggressive
C. normal and active
D. normal and passive
E. normal and standby
Answer: A
Explanation
A unidirectional link occurs whenever traffic sent by a local device is received by its neighbor but traffic from the neighbor is not received by the local device.
UDLD supports two modes of operation: normal (the default) and aggressive. In normal mode, UDLD can detect unidirectional links due to misconnected interfaces on fiber-optic connections. In aggressive mode, UDLD can also detect unidirectional links due to one-way traffic on fiber-optic and twisted-pair links and to misconnected interfaces on fiber-optic links.
Question 3While 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?
A. macros
B. errdisable autorecovery
C. IP Event Dampening
D. command aliases
E. Bidirectional Forwarding Detection
Answer: B
Explanation
When unidirectional link occurs, UDLD can put that port into errdisable state (same as shutdown). The administrator must manually shut/no shut to bring that interface up. If we want the interface to automatically recover then configure the “errdisable autorecovery”. For example:
| errdisable recovery cause udld errdisable recovery interval 30 |
By doing so, the port will be place back in up state (no err-disabled state) after 30 seconds, if the port still has violation it will be placed again in “err-disabled” state, otherwise it will remain in up state.
Question 4After 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?
A. Normal UDLD operations that prevent traffic loops.
B. UDLD port is configured in aggressive mode.
C. UDLD is enabled globally.
D. UDLD timers are inconsistent.
Answer: B
Explanation
UDLD aggressive mode is disabled by default. Configure UDLD aggressive mode only on point-to-point links between network devices that support UDLD aggressive mode. With UDLD aggressive mode enabled, when a port on a bidirectional link that has a UDLD neighbor relationship established stops receiving UDLD packets, UDLD tries to reestablish the connection with the neighbor. After eight failed retries, the port is disabled.
Question 5After 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?
A. The port is fully operational and no known issues are detected.
B. The bidirectional status of “unknown” indicates that the port will go into the disabled state because it stopped receiving UDLD packets from its neighbor.
C. UDLD moved into aggressive mode after inconsistent acknowledgements were detected.
D. The UDLD port is placed in the “unknown” state for 5 seconds until the next UDLD packet is received on the interface.
Answer: A
Question 6
A question about UDLD.
| switch(config)#udld enable switch(config)#udld … switch(config)#int f0/5 switch(config-if)#udld port aggressive |
What happens if interface f0/5 has been damaged?
A. Other interface will recover
B. All links go down
C. Link stays up
Answer: A
Question 7
What is the benefit of UDLD?
A. provides backup for fiber
B. help in preventing loops
C. removes loops
D. determines switch path
Answer: B
Question 8
Which two statements about UDLD aggressive mode are true? (Choose two)
A. UDLD attempts to re-establish a downed link one time before declaring the link down.
B. UDLD message are sent every 15 seconds.
C. If a unidirectional link is detected, the port state is determined by spanning tree.
D. UDLD automatically chooses the messaging interval based on the configured timers.
E. The UDLD detection timer is four times the message interval by default.
F. If a unidirectional link is detected, the port state is changed to errdisable.
Answer: B F
Question 9
A network engineer is considering implementing UDLD throughout the network. Which option must the network engineer consider?
A. UDLD works at Layer 1 of the OSI model
B. UDLD is already enabled by default on all ports on Cisco switches
C. UDLD aggressively disables the port after eight failed retries to connect to neighbor
D. UDLD is an IEEE standard that can be configured on non-Cisco devices
Answer: C
Explanation
The Cisco-proprietary UDLD protocol monitors the physical configuration of the links between devices and ports that support UDLD. UDLD detects the existence of unidirectional links. When a unidirectional link is detected, UDLD puts the affected port into the errdisabled state and alerts the user. UDLD can operate in either normal or aggressive mode.
UDLD is a Layer 2 protocol that works with the Layer 1 protocols to determine the physical status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected LAN ports. When you enable both autonegotiation and UDLD, Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.
UDLD aggressive mode is disabled by default. Configure UDLD aggressive mode only on point-to-point links between network devices that support UDLD aggressive mode. With UDLD aggressive mode enabled, when a port on a bidirectional link that has a UDLD neighbor relationship established stops receiving UDLD packets, UDLD tries to reestablish the connection with the neighbor. After eight failed retries, the port is disabled -> C is correct.
Question 10a
Refer to exhibit.
| errdisable recovery cause udld errdisable recovery interval 15 interface Gi0/1 udld port aggressive |
One pair of fiber that is connected to Gi0/1 has been damaged. What is likely to happen?
A. The interface is shut down until the fiber pair is replaced
B. The interface is prevented from causing spanning-tree loops
C. After the fiber pair is replaced, the interface recovers within 15 minutes
D. The interfaces actively tries to fix the damaged fiber link
Answer: B
Explanation
With this configuration when the damage takes place, the link will be put into err-disabled state but after 15 seconds it will be brought to up state to check if the damage has been fixed. If not it will be brought back to err-disabled state again.
Question 10bA switch has a pair of fiber and it has been damaged. Which two statements is true about the configuration below? (Choose two)
| errdisable recovery cause udld errdisable recovery internal 15 interface gig0/0 udld mode aggressive |
A. It would prevent loops.
B. It would be in shutdown mode until fiber is replaced.
C. It would be up for 15 minutes after fiber is fixed.
D. The port would try to actively repair the damaged fiber after 15 seconds.
Answer: A D
Explanation
When UDLD detects a unidirectional link, it administratively shuts down the affected port and alerts you. Unidirectional links can cause a variety of problems, including spanning-tree topology loops -> A is correct.
With the two first commands, the port will be place back in up state (no err-disabled state) after 15 seconds, if the port still has violation it will be placed again in “err-disabled” state, otherwise it will remain in up state -> D is correct.
Reference: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9500/software/release/16-8/configuration_guide/ha/b_168_ha_9500_cg/b_168_ha_9500_cg_chapter_011.html#concept_xbc_5xx_31bCommentsComment pages« Previous12
- RED1August 16th, 2018I passed today, successfully
I suggest you to review UDLD carefully
I had 40 questions and 6 questions about UDLD - cidkNovember 21st, 2018good.
- MoepMoepDecember 19th, 2018Question 8Question about UDLD aggressive. (Choose two)A. UDLD messages are sent every 15 seconds
B. UDLD tries to reestablish link once before declaring the link down
C. When unidirectional fail is detected, port state is determined by spanning tree
D. When unidirectional fail is detected, port is turned to errdisable
E. UDLD sends messages four times the message interval by default
F. UDLD automatically … message interval based on configured timersAnswer should be B DSource:
https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/12-2/31sga/configuration/guide/config/udld.html#wp1037814 - AnonymousJanuary 11th, 2019