TB - GNS3 Lab: UDLD

Simulating UDLD Failure in GNS3 Without MAC ACL Support

You can find the lab here: (https://gns3vault.com/switching/switch-svi-interface-and-routing)

🔍 Objective

In this lab, we simulate a unidirectional link failure using UDLD (Unidirectional Link Detection) in Cisco switches within GNS3. Our goal is to:

• Configure UDLD in aggressive mode on both ends of a link.
• Simulate a unidirectional communication failure.
• Observe how UDLD detects the failure and shuts down the interface.

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🧪 Lab Topology

• SW1 and SW2 connected via GigabitEthernet0/0
• Testing UDLD on interface GigabitEthernet0/0 between the two switches

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🔧 Step 1: Enable UDLD in Aggressive Mode

On both SW1 and SW2:

SW1(config)# udld enable
SW1(config)# interface gi0/0
SW1(config-if)# udld port aggressive
SW1(config-if)# no shutdown

SW2(config)# udld enable
SW2(config)# interface gi0/0
SW2(config-if)# udld port aggressive
SW2(config-if)# no shutdown

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🚧 Step 2: Attempt MAC ACL Filtering (And Why It Failed)

I originally tried to simulate a unidirectional failure by filtering UDLD packets using a MAC access-list:

SW2(config)# mac access-list extended BLOCK_UDLD
SW2(config-mac-acl)# deny host 0100.0ccc.cccc any
SW2(config-mac-acl)# permit any any
SW2(config)# interface gi0/0
SW2(config-if)# mac access-group BLOCK_UDLD in

However, when we tried to verify it with:

SW2# show mac access-list BLOCK_UDLD

We got:

% Invalid input detected at '^' marker.

💡 Conclusion: The IOSvL2 switch image in GNS3 does not support MAC ACLs properly.

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🔁 Step 3: Simulate the Link Failure Another Way

Since MAC filtering failed, we used a more reliable method: manually disabling one side of the link.

✔ Method: Shut Down the Port on SW2

SW2(config)# interface gi0/0
SW2(config-if)# shutdown

Leave SW1’s port up. After a few seconds, SW1 (running UDLD in aggressive mode) detects the failure:

%UDLD-4-UDLD_PORT_DISABLED: UDLD disabled interface gi0/0, unidirectional link detected

Verify with:

SW1# show udld interface gi0/0
SW1# show interface status err-disabled

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🛠 Optional: Enable Auto-Recovery

To automatically recover from UDLD err-disable state:

SW1(config)# errdisable recovery cause udld
SW1(config)# errdisable recovery interval 30

At the bottom of this screenshot we can see there are no more status updates for the err-disabled state:

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📌 Summary

Step	Description
1	Enable UDLD aggressive mode on both switches
2	Attempted MAC ACL filtering — unsupported in IOSvL2
3	Used port shutdown to simulate unidirectional failure
4	Observed UDLD detecting failure and err-disabling the port

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So easy, a caveman can do it.

• GNS3 switch images like IOSvL2 may not support all IOS features (e.g., mac access-group).
• When simulating network behavior, understanding platform limitations is crucial.
• Always have a fallback method — in this case, interface shutdown did the trick.

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Happy labbing 🔧