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Hardware Thread, Bizarre switch problem in Technical; Had quite a bizarre incident happen a day or two ago.... While I was imaging a suite I started to ...
  1. #1
    KarlGoddard's Avatar
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    Bizarre switch problem

    Had quite a bizarre incident happen a day or two ago....

    While I was imaging a suite I started to receive calls from our 'admin block' that users couldn't print, save docs traverse network shares etc...

    I looked into the matter and realised that all network traffic (my ghost session) was being routed to two switches - both 3COM 4200's - in the admin block and in particular 3 ports that had printers attached to them.

    Now this has never happened before, in fact I was imaging without a hitch last Friday so I find it quite bizarre!

    I looked at the Misconfiguration's and Optimisiation report from 3Com's Network Supervisor and that pointed out that 'Spanning Tree Fast Start' was disabled on several ports on the switches in question but not the ports which seemed to be at the root of the problem. Spanning Tree is enabled on all switches and IGMP Snoop is disabled on all switches in school.


    Any ideas?

    Cheers!
    Last edited by KarlGoddard; 20th March 2008 at 08:45 AM. Reason: Spelling

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    DMcCoy's Avatar
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    You will want igmp snooping *on* before you can use multicast filtering.

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    Port fast

    Ideally you want to enable port fast or fast start on ports that connect to workstations and servers. You disable the fast start on ports that connect from one switch to another switch.

    Ash.

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    Nexarius's Avatar
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    Your problem is to do with how multicasting works.

    If switches are not 'clever' enough to detect multicast packets, or are not detecting them because packet snooping is switched off, then your ghost multicast gets sent to every port on the switch. A switch isn't really designed to handle this amount of load, so you end up with nothing being able to talk to anything else on that switch.

    Enabling IGMP snooping should sort the problem out.

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    KarlGoddard's Avatar
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    Firstly, thanks for the replies guys...

    but this is what I can't sus out.. we had no problems at all with ghosting on the network before Monday. The problem only started this week!

    I've even gone through the switch config guide the NM created and in that he states that snoop should be disabled. I know that goes against conventional thought, and that always worked before. In fact this is the 1st occurrence of any issue with multicasting I've had in 3 years of being at the school! The network (normally) works A1.

    It's just plain odd. The NM's back in work next week I think I'll quiz him and see if he can shed some light onto it

    Thanks guys

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    Nexarius's Avatar
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    Well if I turn off snooping on the core switch here, then it does what you're describing and floods the network...

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