Is the OSI model how networks really work? Where does it break down?
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No. It is a teaching and vocabulary tool. The OSI protocol suite itself lost to TCP/IP, and real protocols cross layers all the time. MPLS sits between layers 2 and 3. VXLAN carries a layer 2 frame inside UDP inside IP, so a "layer 2" network rides on layer 4. QUIC rebuilds transport in user space over UDP and includes TLS. BGP is an application on TCP that controls layer 3. I use OSI numbers for communication and think in actual headers.
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Examples that do not fit neatly
- MPLS: a label stack inserted between the Ethernet header and IP. Usually called layer 2.5.
- Tunnels: GRE puts IP inside IP. VXLAN puts an Ethernet frame inside UDP port 4789. IPsec ESP is an IP protocol that carries an entire IP packet. The layer count repeats inside the tunnel.
- QUIC: transport features (streams, loss recovery, congestion control) live in the application over UDP. TLS 1.3 is built into its handshake.
- Layers 5 and 6: in practice they live inside applications and libraries. Few engineers ever name a "session layer" problem.
- Devices: a layer 3 switch, an NGFW reading layer 7 and a load balancer rewriting HTTP headers all span several layers.
Why the model still earns its place
It gives a shared language ("is this a layer 1 or layer 3 problem?"), it orders troubleshooting, and it explains why a device can or cannot see a field: a switch never looks at the TCP port unless a feature tells it to.
They will ask next
What is the outer and inner structure of a VXLAN packet?
Outer Ethernet, outer IP (VTEP to VTEP), outer UDP to port 4789, an 8-byte VXLAN header with a 24-bit VNI, then the original Ethernet frame with its own IP and TCP inside. It adds 50 bytes of overhead for an untagged outer frame, which is why VXLAN underlays run a larger MTU.
Traps
- Defending OSI as literal truth. Seniors are expected to know where it bends.