UDLD Flashcards
What does UDLD stand for?
Uni-Directional Link Detection.
What does UDLD do?
protects STP when a physical malfunction only allows traffic in 1 direction, even though the link shows as up (cisco proprietary)
How does UDLD work?
sends special layer 2 UDLD frames and expects an echo. Both ends must be configured for UDLD
How should UDLD be configured?
the configureable UDLD interval must be less than max age plus two intervals of forward delay
What are the 2 UDLD modes?
normal and aggressive
What is UDLD normal mode?
if a unidirectional link is detected, the port continues normally, but the port is marked as undetermined and a syslog is generated
What is UDLD aggressive mode?
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
How is UDLD configured?
per-port or globally for all fiber-optic ports. Can be enabled globally, but will only affect fiber ports
Define FLPs.
Fast link pulses. When using auto negotiation, these determine speed and duplex.
Which statement about the UDLD protocol is true?
UDLD is a Cisco-proprietary Layer 2 protocol that enables devices to monitor the physical status of links and detect unidirectional failures
Which option lists the modes that are available for configuring UDLD on a Cisco switch?
normal and aggressive
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?
errdisable autorecovery
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?
UDLD port is configured in aggressive mode.
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?
The port is fully operational and no known issues are detected.
where is UDLD enabled?
all fiber-optic links between switches (must be enabled on both ends)
Can loop guard and udld be used together?
yes
can root guard and udld be used together?
yes
What is UDLD?
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)
What type of links is UDLD designed to be used on?
UDLD was originally designed for Fiber-Optic Links. Regular copper normally does not encounter UDLD issues.
What are the modes of UDLD?
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.
What are some useful show commands when verifying UDLD?
Switch# show udld neighbors
Switch# show udld <interface></interface>
How does UDLD work to protect STP?
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.
How does UDLD work on EtherChannel ports?
It exchanges and echos messages on each link independently.
What are the 2 modes of UDLD operation?
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.
Where must you configure UDLD for it to work?
on both ends of the link.
What is a Cisco proprietary method to detect or prevent the unexpected loss of BPDUs?
Unidirectional Link Detection (UDLD).
Define UDLD
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.
