UDLD Questions

[dfelter]

My questions are as follows:

Why is the default UDLD message interval on the Cat 2950/3550 60 seconds when it is 15 seconds on the the Cat4000 and Cat6000? It would seem that UDLD on the Cat 2950/3550 with default STP parameters and UDLD message interval won't prevent a forwarding loop (the situation that UDLD is suppose to prevent).

The discussion of UDLD in http://www.cisco.com/warp/public/473/77.html and other references is lacking. It mentions the UDLD hold time but does not state what the hold time is set to. My observations reveal that the hold time = 3x message interval time. Can anyone validate?

I assume that if you have aggressive mode enabled, the 8 UDLD packets (that are transmitted at a rate of 1 pps) are transmitted following the expiration of the hold time? Thus, aggressive mode actaully takes 8 seconds longer to errdisable a port than normal mode (and with the default UDLD message interval of 15 seconds (on the Cat 4000/6000) takes 53 seconds (3x15+8 ). This incidently is longer than the 50 seconds allotted for STP to transition a port from blocking to forwarding which introduces the possibility of a forwarding loop being introduced for several seconds. Isn't this a problem?

[rfroom]

UDLD message interval is 60 seconds while Aggressive-mode UDLD is 15 seconds. You are correct by noting that UDLD needs to operate in less than 50 seconds to avoid STP loops. The original UDLD was only intended to indentify mis-wired fiber pairs, while Aggressive-mode UDLD was really intended to prevent STP loops. The documentation doesn't really come out and say that. Original UDLD only errDisables a port that is has mismatch CDP neighbors. A-UDLD errDisables on unidirection link.

Yes, the UDLD holdtime is 3x the message interval.

You mentioned these 8 UDLD packets following the expiration timer? I am have never paid attention to them. Also, I believe the port errDisabled in 45 seconds in my tests, I need to double-check. What version of code did you use when seeing the 8 additional packets.

[rfroom]

You are correct by noting that UDLD takes 53 seconds to shut down a port in the absolute worst case when assuming the port goes unidirectional immediately after sending the last UDLD message.

The 8 additional seconds is the timeout interval of 8 seconds. The timeout interval is run in an effort to re-sync the neighbor.

This needs more investigation, but the message interval can be set to as low as 7 seconds as a work-around.

[dfelter]

I never actually observed the 8 UDLD packets. This information came directly from the document in the URL that I reference (Understanding and Configuring the UDLD Feature). Is this discussion correct?

Your assertion that the message interval is the same for both normal and aggressive mode does not appear to be correct. Running shows that UDLD packets are still transmitted at 60 second intervals in both normal and aggressive mode on the 2950. This was tested on a 2950 running 12.1.9. The only way to change the interval at which UDLD packets are transmitted is through the command.

[rfroom]

I saw the same behavior in Native IOS and IOS for the Cat6k and Cat4k respectively and opened a bug on it a few months. I assume this, A-UDLD at 60 seconds, has not been fixed in 2950.

All CatOS uses 15 seconds for A-UDLD. IOS for Cat4k and Cat6k does in the latest images, actually all images that support A-UDLD. So it looks like 2950 and most likely 3550 got left out of this change.

[rfroom]

Forgot to mention, the discussion is accurate. It just does not list caveats for different platforms and IOS versions. It 100% accurate for CatOS. Needs updating for IOS.

[rfroom]

Here is the scoop on the 2950 UDLD timer:

The UDLD message timer interval is not configurable on the 2900/3500 XL

switches. However, all the platforms on 12.1 software release (such as

2950/3550) do have the configurable timer option. The UDLD message interval is NOT always 15 sec on 2900/3500 XL switches. It depends on the how long port is in linkup state. Initially it starts with 1 sec, then moves to 7

sec, and finally 60 sec if not configured otherwise.

[dfelter]

Thanks for all your help!