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Inter-VLAN Routing Operation

Hosts in one VLAN cannot communicate with hosts in another VLAN unless there is a router or a Layer 3 switch to provide routing services. Inter-VLAN routing is the process of forwarding network traffic from one VLAN to another VLAN. Three options include legacy, router-on-a-stick, and Layer 3 switch using SVIs. Legacy used a router with multiple Ethernet interfaces. Each router interface was connected to a switch port in different VLANs. Requiring one physical router interface per VLAN quickly exhausts the physical interface capacity of a router. The ‘router-on-a-stick’ inter-VLAN routing method only requires one physical Ethernet interface to route traffic between multiple VLANs on a network. A Cisco IOS router Ethernet interface is configured as an 802.1Q trunk and connected to a trunk port on a Layer 2 switch. The router interface is configured using subinterfaces to identify routable VLANs. The configured subinterfaces are software-based virtual interfaces, associated with a single physical Ethernet interface. The modern method is Inter-VLAN routing on a Layer 3 switch using SVIs. The SVI is created for a VLAN that exists on the switch. The SVI performs the same functions for the VLAN as a router interface. It provides Layer 3 processing for packets being sent to or from all switch ports associated with that VLAN.

Router-on-a-Stick Inter-VLAN Routing

To configure a switch with VLANs and trunking, complete the following steps: create and name the VLANs, create the management interface, configure access ports, and configure trunking ports. The router-on-a-stick method requires a subinterface to be created for each VLAN to be routed. A subinterface is created using the interface interface_id subinterface_id global configuration mode command. Each router subinterface must be assigned an IP address on a unique subnet for routing to occur. When all subinterfaces have been created, the physical interface must be enabled using the no shutdown interface configuration command. From a host, verify connectivity to a host in another VLAN using the ping command. Use ping to verify connectivity with the host and the switch. To verify and troubleshoot use the show ip routeshow ip interface briefshow interfaces, and show interfaces trunk commands.

Inter-VLAN Routing using Layer 3 Switches

Enterprise campus LANs use Layer 3 switches to provide inter-VLAN routing. Layer 3 switches use hardware-based switching to achieve higher-packet processing rates than routers. Capabilities of a Layer 3 switch include routing from one VLAN to another using multiple switched virtual interfaces (SVIs) and converting a Layer 2 switchport to a Layer 3 interface (i.e., a routed port). To provide inter-VLAN routing, Layer 3 switches use SVIs. SVIs are configured using the same interface vlan vlan-id command used to create the management SVI on a Layer 2 switch. A Layer 3 SVI must be created for each of the routable VLANs. To configure a switch with VLANS and trunking, complete the following steps: create the VLANS, create the SVI VLAN interfaces, configure access ports, and enable IP routing. From a host, verify connectivity to a host in another VLAN using the ping command. Next, verify connectivity with the host using the ping Windows host command. VLANs must be advertised using static or dynamic routing. To enable routing on a Layer 3 switch, a routed port must be configured. A routed port is created on a Layer 3 switch by disabling the switchport feature on a Layer 2 port that is connected to another Layer 3 device. The interface can be configured with an IPv4 configuration to connect to a router or another Layer 3 switch. To configure a Layer 3 switch to route with a router, follow these steps: configure the routed port, enable routing, configure routing, verify routing, and verify connectivity.

Troubleshoot Inter-VLAN Routing

There are a number of reasons why an inter-VAN configuration may not work. All are related to connectivity issues such as missing VLANs, switch trunk port issues, switch access port issues, and router configuration issues. A VLAN could be missing if it was not created, it was accidently deleted, or it is not allowed on the trunk link. Another issue for inter-VLAN routing includes misconfigured switch ports. In a legacy inter-VLAN solution, a misconfigured switch port could be caused when the connecting router port is not assigned to the correct VLAN. With a router-on-a-stick solution, the most common cause is a misconfigured trunk port. When a problem is suspected with a switch access port configuration, use ping and show interfaces interface-id switchport commands to identify the problem. Router configuration problems with router-on-a-stick configurations are usually related to subinterface misconfigurations. Verify the subinterface status using the show ip interface brief command.

4.5.4 Module Quiz – Inter-VLAN Routing