Identifying and understanding these types of interfaces on Cisco switches (and switch modules) is fundamental knowledge. However, it can be difficult to understand when first starting out. Here is a brief introduction on what each are and how they are used on a network.

Switched Ports

Switched ports are generally the default interface mode on a switch and are usually the type of port used when connecting end devices like workstations and servers. They are strictly layer 2 interfaces which cannot be assigned an IP address and are used to forward frames within broadcast domains or VLANs. In addition to trunking protocols, protocols such as port security and STP also operate on switched ports.

Routed Ports

Routed ports are aptly named ports that function much like a port on a router. They are layer 3 interfaces that can be assigned a network address and can be used to route packets between broadcast domains. On hardware that supports it, you can change a switched port into a routed port by issuing the no switchport interface configuration mode command.

Switched Virtual Interfaces (SVIs)

Like routed ports, SVIs are layer 3 interfaces that can be assigned an IP address. Unlike routed ports, however, SVIs are virtual interfaces set in a VLAN and are not tied to a physical interface. This allows them to be reachable by hosts that are connected to switched ports in the same VLAN. This can be a difficult topic to understand for beginning students; especially since SVIs are named using the VLAN they are set in.

To help illustrate the concept of SVIs further, here's an example:

Let's assume that the switch interfaces Fa0/1 and Fa0/2 are assigned to VLAN 20. Because the PCs and the switch's SVI interface Vlan20 are within the same VLAN they are reachable via their respective IPs on their common subnet. From the PCs' perspectives, the Vlan20 interface on the switch looks like just another host on the same network.

In many cases SVIs are used as the default gateway for the hosts in their assigned VLAN in order to route traffic between other VLANs and remote networks. They are also used to provide IP and management services such as Telnet and SSH on switches. For this reason, a default SVI interface, Vlan1, is always present on managed Cisco switches and cannot be deleted. You'll also often find a separate SVI configured in a VLAN reserved for management purposes.

- Brian Brookman


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