What is the key command that must be enabled on a Layer 3 switch for inter-VLAN routing to work, and what happens without it?

The ip routing global configuration command — without it, the Layer 3 switch has SVIs (Switch Virtual Interfaces) with IP (Internet Protocol) addresses but will not route packets between VLANs (Virtual Local Area Networks).

Why this catches people off guard:

• On a router, IP routing is enabled by default
• On a Layer 3 switch, IP routing is disabled by default
• You can create SVIs, assign IP addresses, and everything looks correct
• But hosts in different VLANs still can't ping each other

The fix is one command:

D1(config)# ip routing

How to verify:

D1# show ip route

• Without ip routing: You'll only see connected and local routes for directly attached subnets, but the switch won't forward between them
• With ip routing: The routing table is active, and the switch routes between all SVIs

Tip: If your Layer 3 switch has SVIs configured correctly but inter-VLAN routing isn't working, ip routing is the first thing to check. It's the #1 "forgot this one command" mistake in labs.

Go deeper:

• Layer 3 Switch InterVLAN Routing (Study-CCNA) — calls out ip routing as the enabling step and shows the routing table that depends on it.