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I ask myself that for some days now, and couldn't find a clear answer.

Imagine a network with several rooms, each room with it's own (Layer 2) Switch. Those are then connected in star topology to another switch which is then connected to the router. All Clients get IPs from the same range, all clients are in the same subnet and in the same VLAN.

Do you know a way how to determine, which client is connected to which switch, without involving the switch. E.g only with technologies and mechanisms taking place on the client.

The goal would be to be able to draw automated maps of the client distribution in the network, but, as I said, without involving the switch (for example access it's API or somethign like that).

Regards Me

Christian
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2 Answers2

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I am afraid that you are right: In general, in a "normal" Network (which means: In a Network with just Standard Switches and Routers), there is no way to do so.

Only if the Switches were managed, you could write a script which polls the arp-tables from the Switches, but of course this doesn't work with unmanaged Switches.

Yurik
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I realised that there is no way to do this. The problem is, that a normal network switch, which works on Layer 2 of the OSI model, is not "visible" for tools like traceroute or such.

If you want to traceroute to, lets say: google.com, your traceroute application sends a packet with a TTL of 1 to your default gateway. Your default gateway then decreases the TTL and discards the package, because it has reached a TTL of 0. Because of that, it answers to your computer and your computer can record, from where it received the first answer. It then sends again a package to google.com, but this time with a TTL of 2, and the process begins from start.

Now you can't do with switches what you can do with routers, because (normal layer 2) switches do not know about IP...

Cisco offers a layer2 traceroute utility, but this is limited to the cisco ecosystem and only works with cisco hardware.

So, I fear the answer is, that it is not possible in general.

Christian
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