UDLD timers and 802.1w

[kjbarrass]

with the default udld timers as below. how long does it take for udld to detect a link to go uni-directional the way i understand this is its 45 seconds. with the aging out of 802.1w BPDU of 6 seconds can it be a case that a spanning tree loop would occur before udld error-disables the interface.

Entry 1

---

Expiration time: 31

Device ID: 1

Current neighbor state: Bidirectional

Device name: FOX0141076Z

Port ID: Gi3/1

Neighbor echo 1 device: FOX064904L1

Neighbor echo 1 port: Gi3/1

Message interval: 15

Time out interval: 5

CDP Device name: core-4507

Any feedback appreciated.

Kind Regards

Kev

[mark.edwards]

Hi, 45 seconds does not sound right. The default on a FE port of a 3550 is shown below. As you can see the default message interval is 7 seconds but can be configured between 1-90 seconds (accordinbg to CCO).

Interface Fa0/24

---

Port enable administrative configuration setting: Enabled

Port enable operational state: Enabled

Current bidirectional state: Unknown

Current operational state: Link up

Message interval: 7

Time out interval: 5

No neighbor cache information stored

[kjbarrass]

the udld timers have not been changed manualy on the switch the only difference is we are using udld aggressive mode not sure if that effects the timers.

[ovt]

UDLD timer is 7 sec when UDLD is first configured, then 15 sec. Min on low-end switches = 1 secs and 7 secs on 6500.

I think that UDLD is too slow to completely prevent RSTP loops caused by unidirectional links.

[mark.edwards]

I'm not so sure about this. UDLD uses timers but also uses "Layer 1 mechanisms" to detect failures (see below extract from CCO). Without searching CCO for info on how these excactly work I would not draw any conclusions.

In normal and aggressive modes, UDLD works with the Layer 1 mechanisms to determine the physical status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected interfaces. When you enable both autonegotiation and UDLD, the Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.

[kjbarrass]

thankyou for that, sounds about right for what we are seeing here. RSTP is bringing interfaces from BLK-ALT to FWD before UDLD err-Disables the port causing a short outage on the VLANs in question. with the shortest hello interval of 7 secs (time to detect = 7*3) I cant see how we can fix it without the need to start using loop guard.

Thanks all for the feedback, looks like I need to get back to my test lab :0(

[ovt]

Drop us a line if you find that loopguard can prevent RSTP loops caused by miswiring (I don't have fiber ports at hands to test it).

[kjbarrass]

will let you know how it goes may take me a week to get it tested, as far as i know the loop guard should put the interface into loop-inconsistent if it stops recieving BPDUs as in a uni-directional situation rather than after 6 secs going straight to DESIG FWD.

Kev