Question about UDLD

[m.lammerse]

I'm having trouble figuring when Unidirectional Link Detection (UDLD) would be useful.

Wouldn't Ethernet keep-alives ensure that an interface never stays up if the TX-RX path between two switches is interrupted?

In the fiber-optic case, if the TX-RX path between two switches is severed, wouldn't that link always go down (autonegotiation or not) since carrier is lost at the other end?

I'm more than a little confused..

UDLD is described here:

http://www.cisco.com/en/US/products/hw/switches/ps663/products_configuration_guide_chapter09186a00800ddc26.html

[m-carey]

I'm studying for the 642-811 test, so I will take a stab at this:

Autonegoation occurs at Layer 1, but UDLD occurs at Layer 2. Implementing UDLD will add a secondary troubleshooting helper because you might have a link-light on both your switches because physical access is there, but if the switch does not receive UDLD signals, it shuts down the interface because traffic is not transmitting in a bi-directional format.

It almost sounds like a hello timer, and if the switch does not receive the message back, the port shuts down.

[m-carey]

I'm studying for the 642-811 test, so I will take a stab at this:

Autonegoation occurs at Layer 1, but UDLD occurs at Layer 2. Implementing UDLD will add a secondary troubleshooting helper because you might have a link-light on both your switches because physical access is there, but if the switch does not receive UDLD signals, it shuts down the interface because traffic is not transmitting in a bi-directional format.

It almost sounds like a hello timer, and if the switch does not receive the message back, the port shuts down.