Lab Objective
Enable UDLD in aggressive mode on a fiber link between two switches, simulate a unidirectional fiber failure where only one strand fails, and verify UDLD detects the mismatch and places the affected port into err-disabled state.
Lab Purpose
Loop Guard, covered in the previous lab, protects against a unidirectional failure by relying on the absence of BPDUs. UDLD takes a more direct approach, specifically designed for fiber links where a single strand can fail while the other keeps working, causing traffic to flow in only one direction — a scenario Ethernet's normal link-state detection cannot see at all, since the physical link layer still reports up.
Lab Topology
Switch1 ---- Gi1/0/10 (fiber) ---- Gi1/0/10 ---- Switch2
Fiber link uses separate TX/RX strands,
allowing a single-strand failure to occur
independently of the other directionTask 1: Enable UDLD Aggressive Mode on Both Switches
Configure UDLD aggressive mode on the fiber interface on both switches.
Task 2: Verify Normal UDLD Operation
Confirm both switches report the link as bidirectional and healthy.
Task 3: Simulate a Unidirectional Fiber Failure
Simulate one strand failing (traffic can only flow Switch1 to Switch2, not the reverse), which Ethernet's normal link-state detection would not notice.
Task 4: Verify UDLD Detects the Failure
Confirm UDLD aggressive mode detects the unidirectional condition and shuts down the affected port.
Task 5: Verify the Port Status
Confirm the port shows err-disabled specifically due to UDLD, distinct from other err-disable causes covered in earlier labs.
Solution and Verification
Switch1(config)# interface gigabitethernet1/0/10
Switch1(config-if)# udld port aggressive
Switch2(config)# interface gigabitethernet1/0/10
Switch2(config-if)# udld port aggressiveSwitch1# show udld gigabitethernet1/0/10
Interface Gi1/0/10
---
Port enable administrative configuration setting: Enabled / in aggressive mode
Current bidirectional state: Bidirectional
Neighbor state: Bidirectional
-- Normal, healthy state before the
-- simulated failure-- Simulated: one fiber strand fails,
-- Switch2 can still send to Switch1, but
-- Switch1's transmissions never reach
-- Switch2 (or vice versa, depending on
-- which strand failed)
-- Notably, the interface itself would
-- normally still show "up/up" from a basic
-- link-state perspective, since fiber
-- optics can maintain a light signal on
-- one strand even if the other has failedSwitch1# show udld gigabitethernet1/0/10
Interface Gi1/0/10
---
Current bidirectional state: Unidirectional
-- UDLD's periodic hello exchange detected
-- that its own hellos are not being
-- acknowledged back by the neighbor,
-- something plain link-state monitoring
-- could never reveal on its ownSwitch1# show interfaces gigabitethernet1/0/10 status
Gi1/0/10 err-disabled
Switch1# show interfaces gigabitethernet1/0/10 | include err-disable
Interface Gi1/0/10 is down (UDLD Port Disabled)
-- Confirms this specific err-disabled state
-- was caused by UDLD, distinct from the
-- port security or BPDU Guard err-disable
-- causes covered in earlier labsKey Takeaway
UDLD aggressive mode solves a blind spot that neither basic Ethernet link-state detection nor Spanning Tree can address on their own: a fiber link that appears physically up and even passes some traffic, but is genuinely broken in one direction — a failure mode that could otherwise allow a Layer 2 loop to form silently, or cause intermittent, hard-to-diagnose connectivity issues depending on which direction of traffic happens to be affected.