Nowadays we have multilayer switch and pure layer 3 device.
Do "pure" layer 3 device like a router use layer 2 protocol ? Do they require layer 2 header to operate ?
If so, what are the differences between multilayer switch and pure layer 3 device ?
1 Answer
Any service in the network layer needs to use the data link layer. In turn, any service in the data link layer needs to use the physical layer.
If the router is a Layer 3 device, does it even support Layer 2 forwarding (like a switch does)?
No. Each router port belongs to a different IP subnet/L2 segment and you don't want MAC-based forwarding or broadcasting between those.
Do "pure" layer 3 device like a router use layer 2 protocol ? Do they require layer 2 header to operate ?
They use and require layer 2 but don't forward based on L2 addresses.
If so, what are the differences between multilayer switch and pure layer 3 device ?
A multi-layer switch provides both, layer-2 (frame-level switching within an L2 segment/subnet) and layer-3 services (packet routing between subnets). A pure layer-3 device only provides packet routing.
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Even if the layer 3 device use layer 2 protocol for L3 forwarding. The mechanism is different. It terminates the Ethernet frame on one interface, and creates a new one on the outgoing interface. The L2 forwarding do not have the "termination" mechanism when L2 forwarding.jacky chong– jacky chong2025-11-14 08:21:14 +00:00Commented Nov 14 at 8:21
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1Sure - L3 forwarding aka routing forwards packets, not frames. FWIW, L2 forwarding terminates the physical-layer stream (L1) and starts a new one, somewhat similar to L3 and L2.2025-11-14 09:34:07 +00:00Commented Nov 14 at 9:34