Implementing and Operating Cisco Enterprise Network Core Technologies (ENCOR) v1.2InfrastructureMedium
A network administrator is troubleshooting an intermittent connectivity issue between two devices on a Layer 2 network. The administrator suspects a unidirectional link failure, where traffic flows in one direction but not the other. This type of failure can lead to silent packet drops and unexpected behavior in protocols like Spanning Tree. Which Cisco proprietary feature helps detect and manage these unidirectional links?
- ABPDU Guard
- BUDLD
- CRoot Guard
- DLoop Guard
Show answer & explanationAnswer & explanation
Correct answer: B. UDLD
UDLD (UniDirectional Link Detection) is a Cisco proprietary protocol that monitors the physical wiring of a link and detects if it is operating unidirectionally. This is crucial because unidirectional links can cause Layer 2 protocols like STP to malfunction, leading to loops or black holes.
Why the other options are wrong
- A. BPDU Guard shuts down a port if it receives a BPDU, typically used on access ports to prevent unauthorized switches.
- C. Root Guard prevents a port from becoming a root port, protecting the STP topology from unauthorized root bridge changes.
- D. Loop Guard is an STP enhancement that prevents alternate or root ports from transitioning to forwarding state if they stop receiving BPDUs, but it doesn't detect physical unidirectional links.
UDLD
UniDirectional Link Detection (UDLD) is a Cisco proprietary Layer 2 protocol that monitors the physical integrity of a link to ensure bidirectional communication, detecting and disabling unidirectional links that can disrupt network protocols.
- Cisco proprietary.
- Detects unidirectional links.
- Prevents STP issues (loops, black holes).
- Operates in normal or aggressive mode.
- Monitors physical wiring.
Memory trick: UDLD sees if flow goes both ways, preventing silent network disarray.