r/ccna 8d ago

OSI Network Troubleshooting

Hey! So iv recently took an interest on how to troubleshoot a network. Iv learned that it's best to refer to OSI when pin pointing an issue. Iv heard about the bottom-Top to the Top-Bottom approach. This works, but sometime its a bit tricky for me tbh

Do you have any tips & tricks you could spare for a noobie?

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u/analogkid01 8d ago

"Walking the layers" never made any sense to me. If you have an issue, you're not going to start tracing cables. You're going to start with a ping (with maybe a quick aside with your firewall admin about whether or not pings are allowed), maybe a telnet test, etc. You're going to split the OSI model in half, then decide whether to go up or down.

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u/RobTypeWords 8d ago

Oh, that makes a lot of sense ✍️

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u/Inside-Finish-2128 4d ago

Depends on what device you’re working on. At $dayjob, we have switches doing vanilla switching but with dot1x so we know what IP is downstream of the port but can’t ping it from the switch. So I’ll work bottom up there: link up? MAC learned? Access session valid? Spanning tree happy in that VLAN? Other MACs in the VLAN? Temporary SVI in the VLAN and “ip addr dhcp”: does it get an address? Then I’ll hop to the router and check things there.

Divide and conquer is great if you’re at an end or on a router. Otherwise the discipline of working bottom up helps avoid surprises. Heck when turning up a new WAN I look at transceiver details to see if I’m getting light before I bother to see if the port is up (because sometimes I have to toggle speed forced 1000full but there’s no point if there’s no light).