Understanding the UDLD Protocol
What is UDLD?
The Unidirectional Link Detection (UDLD) protocol is a Layer 2 (L2) protocol designed by Cisco to detect unidirectional links before they create forwarding loops. UDLD works in conjunction with Layer 1 (L1) mechanisms to monitor the physical status of a link.
How UDLD Works
- L1 and L2 Coordination: While auto-negotiation at L1 handles physical signaling and fault detection, UDLD at L2 ensures logical connectivity.
- Protocol Packet Exchange: UDLD-enabled ports send packets containing device and port IDs to their neighbors.
- Echo Mechanism: Neighboring ports must see their own IDs echoed back in incoming packets. If they don’t, the link is considered unidirectional and is shut down.
Enabling UDLD
To enable UDLD, it must be supported and activated on both devices on a link. This dual-layer protection helps prevent both physical and logical unidirectional connections, enhancing network stability.
Reference: http://www.cisco.com/warp/public/473/77.html