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Wired Networks Thread, Duplicate IP causing loopback-like conditions? in Technical; Silly question from someone that really should know. Flat network. Nothing special involved. All devices in one (brand new) switch. ...
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    synaesthesia's Avatar
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    Duplicate IP causing loopback-like conditions?

    Silly question from someone that really should know.

    Flat network. Nothing special involved. All devices in one (brand new) switch. Did it on the old one as well.
    Conditions are like a loopback - i.e. pinging from any machine to any other is showing replies between 1ms and 1s, if at all.

    The only thing I can think of that might be causing this, is that the school has a fair few tablets/phones that are connected which have been given static IP addresses by site-staff (with little advice from me) in part of the IP scope which allows a lower level of internet filtering.


    I'm gonna run wireshark on it the next time I'm there, but for the time being we've just written off the wireless (hodge-podge APs) to try and get an idea of what's going on. Last time it happened we thought it was a dodgy AP so pulled it out and it was fixed. If it's a wireless client then that's obviously more likely to be a cause, obviously fixed when the AP is no longer there. Just trying to think of the most logical starting point!

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    m25man's Avatar
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    Make sure that you have excluded the static device range from the scope otherwise DHCP will still hand them out, then your static device wanders back in and screws your life up for the afternoon..

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    synaesthesia's Avatar
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    They were well out of the scope anyway. One thing I did today to help some confusion was to expand the scope and add exceptions & reservations. Staff were going home and having to change settings on devices to DHCP instead of their statics which was daft.
    It's likely because they haven't been keeping proper check on what has what address. That part of it is pretty much solved when we get hold of all the tablets etc but for now I'm wondering if something that simple can cause such a large problem network-wide.

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    VLAN the wireless devices would be a good start IMO.

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    Turn STP off on the switch see if that helps?

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    synaesthesia's Avatar
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    Not vlanning a flat switch, no need to complicate matters when this single switch runs 90% of the entire school with no edges!
    Will check out STP - however it was happening previously on an old 3com 4400 which doesn't.

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    Quote Originally Posted by synaesthesia View Post
    They were well out of the scope anyway. One thing I did today to help some confusion was to expand the scope and add exceptions & reservations. Staff were going home and having to change settings on devices to DHCP instead of their statics which was daft.
    It's likely because they haven't been keeping proper check on what has what address. That part of it is pretty much solved when we get hold of all the tablets etc but for now I'm wondering if something that simple can cause such a large problem network-wide.
    Absolutely, a single misconfigured device will generate gratuitous ARP and broadcast traffic and it only takes one device with an IP conflict to bring your small flat network to its knees.
    DHCP everything exclude absolute ranges, use reservations, that way your clients can take the devices home and connect to their network without messing about with setting that can kill your LAN when they return.
    I cant believe what we find sometimes when I plug the Etherscope on so called "Good" networks, misconfigured gateways, masks, duped IP's and devices with static addresses well off range.
    10 minutes with the scope I walk out the hero when in fact the problem was so stupidly simple but almost impossible to trace without insight.
    I had one school where a teacher was connected to the schools wifi on her Samsung phone and had a static IP the same as the schools DC!
    Anything is possible and the simpler the network the more likely it is to happen.

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