Inter-VLAN Routing

• Module Title: Inter-VLAN Routing
• Module Objective: Troubleshooting inter-VLAN routing on Layer 3 devices

Inter-VLAN Routing Operation

• VLANs segment Layer 2 networks, but hosts in different VLANs require a router or Layer 3 switch for communication
• Inter-VLAN routing forwards network traffic between VLANs
• Inter-VLAN routing options consist of legacy, router-on-a-stick, and Layer 3 switches using switched virtual interfaces (SVIs)
• Legacy Inter-VLAN routing uses a router with multiple Ethernet interfaces connected to switch ports in different VLANs
• Each router interface acts as the default gateway for local hosts on its VLAN subnet
• Requiring one physical router interface per VLAN limits scalability due to the finite number of router interfaces

Router-on-a-Stick Inter-VLAN Routing

• Overcomes limitations by using one physical Ethernet interface to route traffic between multiple VLANs
• A Cisco IOS router Ethernet interface is configured as an 802.1Q trunk and connected to a Layer 2 switch trunk port
• Router interface is divided into subinterfaces to identify routable VLANs
• Configured subinterfaces are software-based virtual interfaces, each linked to a single physical Ethernet interface
• Each subinterface receives an IP address and VLAN assignment, aligning with different corresponding subnets to facilitate logical routing.
• When VLAN-tagged traffic enters the router interface, it goes to the VLAN subinterface
• The router decides on the exit interface based on the destination IP network address, adding a new VLAN tag to the frames if needed, before sending them out the physical interface
• Does not scale beyond 50 VLANs

Inter-VLAN Routing on a Layer 3 Switch

• Employs Layer 3 switches and SVIs for modern inter-VLAN routing
• An SVI is a virtual interface configured on a Layer 3 switch
• A Layer 3 switch operates at both Layer 2 and Layer 3
• Inter-VLAN SVIs are created like management VLAN interfaces
• SVIs provide Layer 3 processing for packets sent to/from switch ports in their VLAN
• Advantages of Layer 3 switches for inter-VLAN routing include greater speed due to hardware-based switching and routing
• External routing links are not necessary when using Layer 3 switches
• Layer 2 EtherChannels can be used as trunk links between the switches to increase bandwidth
• Lowers latency because data does not need to leave the switch to be routed to a different network
• More commonly deployed in a campus LAN than routers
• Disadvantage: More expensive

Router-on-a-Stick Inter-VLAN Routing Configuration

• S1 switch configuration steps are: create/name VLANs, create management interface, configure access ports, configure trunking ports
• Router-on-a-stick involves creating a subinterface for each VLAN to be routed using the command interface interface_id subinterface_id
• Match the subinterface number with the VLAN number

Router Subinterface Configuration

• The two commands to configure each subinterface are:
  • encapsulation dot1q vlan_id [native]: responds to 802.1Q encapsulated traffic from the specified vlan-id
  • ip address ip-address subnet-mask: configures the IPv4 address of the subinterface.
• The router subinterface must be assigned an IP address on a unique subnet for routing to occur
• Enable the physical interface with the command no shutdown interface

Router-on-a-Stick Inter VLAN Routing Verification

• To verify connectivity to a host in another VLAN, use the Ping command
• To verify the current host IP address use the command prompt ipconfig
• The following show commands can be used to verify the router-on-a-stick configuration:
  • show ip route
  • show ip interface brief
  • show interfaces
  • show interfaces trunk

Layer 3 Switch Inter-VLAN Routing

• Enterprise campus LANs use Layer 3 switches to provide inter-VLAN routing
• A Layer 3 switch can:
  • Route from one VLAN to another using multiple switched virtual interfaces (SVIs).
  • Convert a Layer 2 switchport to a Layer 3 interface, a routed port
• Create an SVI for each routable VLAN
• Steps to configure S1 with VLANs and trunking: create VLANs, create the SVI VLAN interfaces, configure access ports, and enable IP routing

Layer 3 Switch Inter-VLAN Routing Verification

• Testing connectivity between hosts after configuration verifies inter-VLAN routing using a Layer 3 switch
• Using the ping command from a host to verify connectivity and using the ipconfig Windows host command enables checking current host IP configuration
• To enable routing on a Layer 3 switch, a routed port must be configured
• A routed port is created by disabling the switchport feature using the no switchport interface configuration command on a Layer 2 port

Routing Configuration on a Layer 3 Switch

• Steps to configure a Layer 3 switch to route:
  • Configure the routed port
  • Enable routing, then use the ip routing global config command to enable routing
  • Configure routing using the correct method(Single-Area OSPFv2)
  • Verify routing
  • Verify connectivity.

Common Inter-VLAN Issues

• Issues are generally related to connectivity problems
• First look into the physical layer for issues with cabling. Is it in the right port?
  • Missing VLANs
    • Create a VLAN and ensure that it is assigned to the correct VLAN
    • Use the command prompt show vlan [brief]
  • Switch Trunk Port Issues -Ensure trunks are configured correctly and that the port is enabled as a trunk port
    • Use the command prompt show interface trunk
  • Switch Access Port Issues
    • Assign correct VLAN to access port and that it is enabled
    • Use the command prompts show interfaces switchport show running config interface and ipconfig
  • Router Configuration Issues
    • Check router subinterface IPv4 and that it is assigned to the VLAN ID.
    • Use the command prompts show ip interface brief and show interfaces

What was learned

• Inter-VLAN routing options. Can use legacy, router-on-a-stick, and Layer 3 switch using SVIs.
• To configure a switch with VLANs and trunking do the following: create and name the VLANs, create the management interface, configure access ports, and configure trunking ports.
• A subinterface is created using the command prompt: interface interface_id subinterface_id
• the physical interface must be enabled, with the command prompt: no shutdown interface
• use Layer 3 switches to provide inter-VLAN routing.
• SVIs. SVIs are configured using the same interface vlan vlan-id command used to create the management SVI on a Layer 2 switch.
• create the SVI VLAN interfaces, configure access ports, and enable IP 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 device can be configured with IPv4 to connect to a router or another Layer 3 switch.
• 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.
• VLAN, misconfigured switch and router ports. Ensure the connecting router port is not assigned to the correct VLAN. or the trunk.
• use ping and show interfaces interface-id switchport commands to identify the problem. Verify the subinterface status using the show ip interface brief command.

Learn about troubleshooting inter-VLAN routing on Layer 3 devices. Discover legacy methods, router-on-a-stick, and Layer 3 switches using SVIs. Understand how one physical interface can route traffic between multiple VLANs.