18. VLANs Part 3

Native VLAN on a Router (ROAS)

• Native VLAN untagged frames are more efficient and faster than tagged frames because they are smaller in size.

• Let’s reset both SW1 and SW2 to native VLAN 10.

  

Configuring Native VLAN on a Router

There are two methods to configure the native VLAN on a router:

1. Using Sub-Interface:

   • Use the command encapsulation dot1q <vlan-id> on the router’s sub-interface.

     

2. Using the Physical Interface:

   • Configure the IP address directly on the router’s physical interface.

   • The encapsulation dot1q <vlan-id> command is not required.

     

Show Running Configuration

• Example output of the show running-config command for the G0/0 interface:

  

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Layer 3 (Multilayer) Switches

• A Multilayer Switch can perform both switching and routing functions.

• It operates at Layer 3 of the OSI model (Network Layer), meaning it is capable of routing IP packets.

• You can assign IP addresses to Layer 3 virtual interfaces on a Multilayer Switch, similar to a router.

• SVIs (Switch Virtual Interfaces) allow you to create virtual interfaces for each VLAN and assign them IP addresses.

• The switch can perform inter-VLAN routing, eliminating the need for a router in some network designs.

  

Replacing SW2 with a Layer 3 Switch

• The multi-VLAN connections to R1 are replaced with a point-to-point Layer 3 connection.

  

Using SVIs for Routing

• Configure each host to use the SVI (not R1) as the gateway address.

• When hosts need to send traffic to different subnets or VLANs, they send traffic to the switch, which performs the routing.

  

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Clearing R1’s Configuration

• To work with the Layer 3 point-to-point connection, clear the sub-interface configuration on R1:

  • #no interface <sub-interface id> — Removes the VLAN interface.
  • #default interface g0/0 — Resets the G0/0 interface to its default settings.
  • Then configure the default G0/0 interface on R1 with the IP address 192.168.1.194 as shown in the network diagram.

  

Configuring SW2 for Layer 3 Routing

• Set up SVIs on SW2 and establish the Layer 3 point-to-point connection with R1.

• Key commands:

  • default interface <interface-id> — Resets the specified interface to its default settings.
  • ip routing — Enables Layer 3 routing on the switch.
  • no switchport — Converts the interface from a Layer 2 switchport to a Layer 3 routed port.

  

• Configure a default route to R1 (192.168.1.194) so that all traffic leaving the network gets routed through R1’s Gateway of Last Resort.

  

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SVI Configuration on SW2

Switch Virtual Interfaces (SVIs)

• SVIs are the Layer 3 virtual interfaces on a switch that allow routing between VLANs.

• SVIs are shut down by default, so use the no shutdown command to enable them.

  

Troubleshooting SVI Status

• If you create an SVI for an unknown VLAN (e.g., VLAN 40), the status/protocol will show “down/down”.

  

To bring an SVI to an “up/up” state, ensure the following:

• The VLAN must exist on the switch.

• At least one access port in the VLAN must be in an up/up state or a trunk port that allows the VLAN must be in an up/up state.

• The VLAN itself must not be shutdown (use the no shutdown command if needed).

• The SVI must also be enabled (no shutdown).

  

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Final Setup: Layer 3 Switch SVI Trunk Replacement

• The VLAN trunk has been successfully replaced by a Layer 3 Switch SVI.
• All hosts can now communicate with each other (tested using ping) and access the external internet via R1.

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