udld & udld agressive

[subhojithalder198]

Hi Team,

We have 2 no Cisco switches connected over Fiber, I am having below query on udld mode

as per my understanding this are the details

In “Normal” mode, if the physical state of port (as reported by Layer 1) is still up, UDLD marks this port as “Undetermined”, but does NOT shut down or disable the port, which continues to operate under it’s current STP status. This mode of operations is informational and potentially less disruptive (though it does not prevent STP loops).

If UDLD is set to “Agressive” mode, once the switch loses it’s neighbor it actively tries to re-establish the relationship by sending a UDLD frame 8 times every 1 second  . If the neighbor does not respond after that, port is considered to be unidirectional and brought to “Errdisable” state.

Pls confirm is my understanding correct or wrong.

Also confirm what is the cisco recomended version on the same, need to use udld or udld-aggressive

Also confirm whether we need to use loop guard in this situation as per my understading, udld only help us for physical failure

Br/Subhojit

[Rolf Fischer]

Hi Subhojit,

unfortunately, Cisco's UDLD documentation is not very clear.

In “Normal” mode, if the physical state of port (as  reported by Layer 1) is still up, UDLD marks this port as  “Undetermined”, but does NOT shut down or disable the port, which  continues to operate under it’s current STP status.

That's what I always thought too after reading the documentation.

But then I experienced that ports in normal mode can turn to err-disable as well:

https://supportforums.cisco.com/message/3712873

To my best knowledge, the detection mechanism is the main difference between the two modes:

When an aggressive port looses (UDLD-)connectivity, it actively tries to re-establish the connection (as you described).

A normal port, in contrast, will be err-disabled (timer-based) when it receives UDLD hellos from its neighbor, but its own ID is not reported in those hellos (meaning that the neighbor is not aware of the neighborship).

HTH

Rolf