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Wired Networks Thread, Possible Inter V-Lan Routing Issue in Technical; Morning Guys, We have a strange issue here potentially with our inter-vlan routing. Basically when trying to connect to something ...
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    Possible Inter V-Lan Routing Issue

    Morning Guys,

    We have a strange issue here potentially with our inter-vlan routing. Basically when trying to connect to something on another v-lan the first time it will come back as not being able to find it but subsequent times it will connect fine.

    The annoying thing is it doesn't seem to do it all the time.

    Any ideas?
    Last edited by Sam_Brown; 12th May 2014 at 10:21 AM.

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    What sort of switches do you use?

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    HP5120 and HP5800

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    Not sure on HP sorry. Cisco is that I deal with.

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    Hi Sam,

    Did you get to the bottom of this ?

    If it helps, it sounds like you are having some issues with ARP. Its not uncommon to loose a ping or two when trying to talk to a box for the first time on a LAN, this is just the ARP protocol doing its stuff, but the loss of one or two pings (we are talking about 10's of miliseconds) shouldn't cause a box to fail to respond consistently as you describe.

    Try pinging a misbehaving box and note any packet loss and RTT times, on a LAN you should be seeing RTT's of less than 1ms and you should have no packet loss save from possibly one or two the first time you ping it if it has seen no traffic for a few hours.

    check the layer 3 interfaces for the relevant vlans on your switch and make sure proxy-arp and ip-redirects are turned off, this is particularly important where there is more than one router connected to a subnet, say where you have a split core architecture. These features can cause delays in ARP replies and cause the type of issues you are seeing. Before you do that though, make sure all your servers/pc's etc are configured with the correct subnet masks and gateways as the proxy-arp feature hides issues that would otherwise be caused by devices with no default gateway configured.

    There are other things which could be causing it but this is a good place to start.


    Cheers
    Shaun



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