Section 22: Inter-VLAN Routing

What is a Switched Virtual Interface (SVI), and what does a Layer 3 switch use it for?

A virtual interface on a Layer 3 switch that enables routing between VLANs without needing an external router.

What does RoaS stand for, and what does it require on the router's physical interface?

Router on a Stick — a single physical router interface is divided into virtual sub-interfaces, one per VLAN, rather than needing a separate physical interface for each VLAN.

In Router-on-a-Stick, how does the router know which VLAN a frame belongs to on its single physical interface?

The 802.1Q tag on the trunk specifies which sub-interface (and thus which VLAN) the traffic is for.

What switch port mode is used on the switch side when connecting to a router using separate physical interfaces per VLAN, vs. Router-on-a-Stick?

Separate interfaces per VLAN → switch port is in access mode. Router-on-a-Stick → switch port is in trunk mode.

In interface GigabitEthernet 0/0/1.3, what does the .3 represent, and does it need to match anything?

The sub-interface number — it's arbitrary/locally significant and doesn't need to match the VLAN number or anything on the other side (though matching it to the VLAN number is common practice for clarity)

What command enables IP routing on a Layer 3 switch, and is it on by default?

ip routing — no, it's not enabled by default, even if SVIs are configured.

What command converts a Layer 3 switch's physical interface into a routed port (removing its Layer 2 switching behavior)?

no switchport

Even when using a Layer 3 switch for Inter-VLAN routing, what might still require an external router?

WAN connectivity and certain services.

How does routed traffic within the campus differ between using a Layer 3 switch vs. Router-on-a-Stick/separate-interfaces for Inter-VLAN routing?

With a Layer 3 switch, inter-VLAN traffic is routed across the switch backplane. With RoaS or separate router interfaces, traffic has to physically travel up and down the Ethernet cable to the external router.

Does an OSPF/router sub-interface need its own no shutdown command?

No — only the underlying physical interface needs no shutdown. As long as that's up, sub-interfaces come up automatically

When using a Layer 3 switch for Inter-VLAN routing with a separate external router for WAN connectivity, why does the router need an explicit route back to the internal VLAN subnets?

Because the router is not directly connected to those subnets (the switch is) — unlike the separate-interfaces or RoaS designs, where the router is directly attached to every VLAN subnet