Layer 3 Switch to Router Link

About this interactive

From the lesson: - **The link to the router:** a Layer 3 switch routes between its own VLANs, but reaching the outside world still goes through the router. The link between them is best on a network of its own, a /30, which holds just the two addresses it needs (more control and more security than sharing a user VLAN). - **The router side** is a plain routed port with an address, whichever way the switch side is built. - **The switch side, two ways:** a routed port (the port stops switching and takes the address itself), or an SVI in a VLAN for the link, with the port to the router an access port in that VLAN (not a trunk). - **The way back:** the switch knows every VLAN it routes, but the router knows only its directly connected networks. Until it has a route back to each VLAN (a static route or a routing protocol), hosts behind the switch get no replies from it.

What you'll learn

Predict whether a Layer 3 switch and its router can reach each other over a routed port or an SVI link on a /30 network of its own
Explain why hosts behind a Layer 3 switch get no replies from the router until the router has a route back to their VLANs

Aligned to

Cisco CCNA

2.1 Configure and verify VLANs (normal range) spanning multiple switches

1.6 Configure and verify IPv4 addressing and subnetting

3.1 Interpret the components of routing table

3.2 Determine how a router makes a forwarding decision by default

Key terms

Layer 3 Switch: A network switch that incorporates routing functionality, allowing it to forward traffic based on both MAC addresses and IP addresses across multiple networks.
Routed Port: A switch port on a multilayer switch that has been configured to operate like a router interface at Layer 3 rather than as a switching port, allowing it to be assigned an IP address and participate in routing protocols. Routed ports are used where a Layer 3 boundary is needed directly on the switch without a separate router.
Switch Virtual Interface: A virtual layer 3 interface on a switch associated with a VLAN. On a layer 2 switch a single SVI provides the management IP address; on a layer 3 switch, SVIs also route between VLANs.
Access Port: A switch port that belongs to a single data VLAN and sends and receives untagged frames, used for connecting end devices such as PCs and printers. An access port may additionally carry a tagged voice VLAN for an IP phone without becoming a trunk.
Static Route: A routing table entry manually configured by an administrator that specifies a fixed path to a destination network and never changes unless the administrator modifies it. Static routes are simple and predictable but do not adapt automatically to network topology changes.
Routing Table: A database stored in a router that lists known network destinations, the metric (cost) to reach them, and the next-hop address or outgoing interface to forward packets toward those destinations. Routers consult this table for every packet they forward.

Topics

Inter Vlan Routing Layer 3 Switch Routed Port Svi Static Route Interactive Predict

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