Lectures on LAN Switching - Part 10

Other Port Types: L3 port and SVI

We've already discussed Access and Trunk ports since they're the commonly used port types for Switches' physical interfaces (or ports). As our next topic, we're going to discuss a bit about Layer-3 (L3) ports, as well as Switch Virtual Interfaces (SVI) or more commonly known as VLAN Interfaces. These port types are generally available on Switches with routing or Layer-3 capabilities, such as:

• Catalyst 3550
  
• Catalyst 3560
  
• Catalyst 3750
  
• and most Modular Catalyst Switches

Layer-3 (L3) Port Type

L3 ports are basically routed ports. Meaning, these are previously "switched" ports that are configured to be similar to router ports. You'll want to connect this type of port to a router or to another routed port.

One main difference with real router ports is that L3 ports in Switches are not capable of having sub-interfaces.

Here's a sample configuration in creating an L3 port:

Switch#config terminal
Switch(config)#int fa0/1
Switch(config-if)# no switchport
Switch(config-if)# ip address 192.168.0.1 255.255.255.0
Switch(config-if)# no shut

You'll notice that the main configuration command is 'no switchport'. This is like literally saying to the port to stop being a "switched" port, and become the opposite (which is to be a "routed" port). The IP address configuration is just one of the most common configuration you'll do in a router port. Lastly, you need to issue a 'no shutdown' command to enable the port since all L3 ports are disabled by default.


Switch Virtual Interface

Switch Virtual Interfaces, or SVIs, are logical interfaces on a Layer-3 capable Switches. These serve as the gateway for the VLANs (or VLAN segments) to communicate with other VLANs. In a way, an SVI is similar to a router port wherein the router port leads to a network segment.

In its most basic function, you configure an IP address to the SVI to serve as the gateway IP address of a VLAN/network segment. Meaning, all PCs connected on ports that belong to a specific VLAN, will use the IP address of the corresponding SVI as their own gateway IP address. Moreover, configuring an IP address to an SVI enables PCs on a VLAN segment to communicate with other PCs on other VLAN segments (with configured IP addresses).

You also need to be careful in referencing SVIs. Since the more common term for SVI is Interface VLAN, a lot of people thought that this also refer to the VLAN itself. To be clear, VLAN is the network segment, while SVI (or Interface VLAN) is the gateway (much like a router interface).

Here's a sample configuration in creating an SVI:

Switch#config terminal
Switch(config)#int vlan 10
Switch(config-if)# ip address 192.168.0.1 255.255.255.0

Simple eh? =) Doing an 'interface vlan 10' effectively creates the logical SVI. To reiterate, putting an IP address effectively opens the "gate" to other VLANs for what we call "InterVLAN routing". Lastly, the IP address also serves as the telnet IP address of the Switch for any PC (on the same VLAN as the VLAN interface) who wishes to access the Switch.