UDLD: Unidirectional Link Detection

🔍 Today’s Topic: UDLD (Unidirectional Link Detection)

🧩 UDLD is a Layer 2 protocol that must be configured on both ends of a link to form a neighbor relationship.

🔄 We use UDLD to make sure the link is bidirectional by sending an ECHO message every 15 seconds ⏱️

📬 If the other switch replies — awesome! ✅

💬 “I see you!”

Everything is working fine 💪

😬 But what if there’s no reply?

Here comes the Aggressive Mode 💣

🧨 After the 15-second hello timer expires with no response, the switch sends 8 more UDLD packets — one every second!

🚨 Still no answer?

👎 Okay… you're fired! 🔥

That link is now unidirectional and will be shut down to protect the network from loops or weird behaviors 🚫🔁

🧪 To verify the UDLD neighbor relationship:

show udld neighbors

show udld gig0/1

If the port is in err-disabled state:

show interface status

🔧 To recover the port, you have two options:

🧑‍🔧 Option 1 – Manual Recovery:

interface gig0/1

shutdown

no shutdown

🤖 Option 2 – Automatic Recovery:

Switch(config)# errdisable recovery cause udld

Switch(config)# errdisable recovery interval 30 ← the port will recover after 30 seconds

💡Conclusion:

UDLD is your Layer 2 bodyguard 🛡️

it ensures both ends of the link are alive and talking.

If one side stops responding, UDLD protects your network from potential trouble. 🔐⚡