I have a 3com Baseline Switch 2928 that's connected over fiber to a 3com superstack 4500. The fiber is singlemode and has the suitable SFP modules on both switches.

The switches (and connected devices) have working networking but it's very unstable. Browsing SMB shares is workable but has lots of 'waiting intervals': sometimes opening a folder takes 20 seconds after which everything is back ok. All devices on the server side have very stable networking, so the issue must be with the baseline 2928 or the fiber SFP module.

Are there any logs I can check or other things to see what could be the problem? The syslog on the 2928 shows no problems. I've tried all the basic things such as change the SFP port, different cables, various OS's, etc, etc.

The fiber should be ok, I have detailed OTDR measurements showing ok splices and connectors.

  • What is the distance/latency between the two switches? CIFS over anything that as more than a few ms of latency is pain in the ass as CIFS (v1) is extremely chatty and does a few round trips for almost any operation. – pfo Jan 12 '12 at 12:46
  • And btw do you span a larg-ish L2 domain over that thing? – pfo Jan 12 '12 at 12:58
  • Splices = Loss, not matter how "good" your OTDR says. Grab another fiber cable and test. Singlemode is capable of carrying 40 cable TV channels over 4+ miles with ~60ms latency, your application is sound, I doubt the cabling. – Tim Jan 12 '12 at 14:04
  • Latency is max 1mx, almost always below that. Sorry but what do you mean with a larg-ish L2 domain? Already test with a different fiber patch cable, different SFP module and other fiber pairs (cable is 48 fibers). – Michael Anckaert Jan 12 '12 at 14:17

Show the interface statistics to see what is going on.

at the CLI enter "sh int 48" where 48 is your fiber port.

Post the results and it should hopefully be obvious.

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