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Silver

STP on dual UTP/Fibre links

We have a core network comprising of two Cat6k switches. For a couple of sites we have chosen to install two LAN Extention-type services terminating in a single switch (usually 2950-series). The LES circuit are provided by the telco and are media converted at both ends from dark fibre onto a UTP presentation; these then connect to our core switches.

The issue is that on a number of occasions, a fault has developed at one of these sites where one of the LES links becomes unidirectional. Since the telco's media converters do not reflect the state of the fibre port in the state of the UTP port (i.e. the fibre port can be unidirectional, or disconnected completely) and the UTP connection remains up. I believe this is causing an STP loop which then proceeds to affect the remainder of the network (HSRP instability and consequential loss of OSPF adjacencies etc.)

I cannot see a simple solution to this problem, apart from perhaps using UDLD in aggressive-mode (no non-aggressive UDLD will only detect a unidirectional link in this scenario, not actively errdisable any port). UDLD aggressive-mode appears to offer a potential solution on the bench, but has other issues. For example, if the remote switch reloads (e.g. after a power failure), then aggressive-mode on the core switch will err-disable the links because the remote end has gone down). This then requires manual intervention to fix.

1 REPLY
Silver

Re: STP on dual UTP/Fibre links

Although aggressive mode is very efficient in breaking redundant paths in case of loops, there are a few drawbacks. Indeed, most of the time, there is no console access on access switches and they may be located on remote locations. So, network recovery may require somebody to go on sites and re-enable each link manually. Normal mode is generally recommended. Aggressive mode may be used in cases where port stuck issues are suspected.

You could also refer to the following document for troubleshooting spanning tree issues.

http://www.cisco.com/warp/public/473/16.html

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