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.
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).
Port enable administrative configuration setting: Enabled
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.
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(
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.
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