Table of Contents

Configuring Virtual LAN Encapsulation

Configuring Virtual LAN Encapsulation

This chapter describes virtual LAN configurations for the Catalyst 2948G-L3 switch router. It describes how to configure Inter-Switch Link (ISL) VLAN encapsulation and how to configure 802.1Q VLAN encapsulation. For more information about the Cisco IOS commands used in this chapter, refer to the Cisco IOS Command Reference publication. This chapter includes the following sections:

• About Virtual LANs
  
  
• Configuring ISL VLAN Encapsulation
  
  
• Configuring 802.1Q VLAN Encapsulation
  
  

About Virtual LANs

Virtual LANs enable network managers to group users logically rather than by physical location. A virtual LAN (VLAN) is an emulation of a standard LAN that allows data transfer and communication to occur without the traditional restraints placed on the network. It can also be considered a broadcast domain set up within a switch. With VLANs, switches can support more than one subnet (or VLAN) on each switch, and give routers and switches the opportunity to support multiple subnets on a single physical link. A group of devices on a VLAN are configured so that they communicate as if they were attached to the same LAN segment, when they are actually located on different segments. Layer 3 switching supports up to 244 VLAN subinterfaces per system.

VLANs enable efficient traffic separation and provide excellent bandwidth utilization. VLANs also alleviate scaling issues by logically segmenting the physical LAN structure into different subnetworks so that packets are switched only between ports within the same VLAN. This can be very useful for security, broadcast containment, and accounting.

Layer 3 switching software supports a port-based VLAN on a trunk port, which is a port that carries the traffic of multiple VLANs. Each frame transmitted on a trunk link is tagged as belonging to only one VLAN.

Layer 3 switching software supports VLAN frame encapsulation through the Inter-Switch Link (ISL) protocol and the 802.1Q standard.

Figure 5-1 shows a network topology where two VLANs span a Catalyst 5500 switch and Catalyst 2948G-L3 switch router. Both VLANs in this topology are bridged using the Inter-Switch Link (ISL) protocol.

Figure 5-1: VLANS Spanning Devices in a Network

Configuring ISL VLAN Encapsulation

Inter-Switch Link (ISL) is a Cisco protocol for interconnecting multiple switches and maintaining VLAN information as traffic travels between switches.

The VLAN configuration example for the Catalyst 2948G-L3 in Figure 5-2 shows the following:

• Fast Ethernet port f1 and subinterface f2.1 on the switch router are in bridge group 1. They are part of VLAN 50, which uses ISL encapsulation.
  
  
• Fast Ethernet port f3 and subinterface f2.2 are in bridge group 2. They are part of VLAN 100, which uses ISL encapsulation.
  
  
• Fast Ethernet port f2 is configured as an ISL trunk.
  
  

Figure 5-2: Example of an ISL VLAN Bridging Configuration

To configure the ISL VLANs shown in Figure 5-2, use the following procedure beginning in global configuration mode.

To monitor the VLANs once they are configured, refer to the "Monitoring and Verifying VLAN Operation" section.

Configuring 802.1Q VLAN Encapsulation

IEEE 802.1Q VLAN encapsulation uses an internal, or one level, packet tagging scheme to multiplex VLANs across a single physical link, while maintaining strict adherence to the individual VLAN domains. You can use 802.1Q encapsulation on the Catalyst 2948G-L3 switch router for routing traffic across the network.

On an IEEE 802.1Q trunk port, all transmitted and received frames are tagged except for those on the VLAN configured as the PVID (port VLAN identifier) or native VLAN for the port. Frames on the native VLAN are always transmitted untagged and are normally received untagged.

The VLAN configuration example for the Catalyst 2948G-L3 switch router shown in Figure 5-3 depicts the following:

• Fast Ethernet subinterface f2.1 is in the 802.1Q native VLAN 1.
  
  
• Fast Ethernet subinterface f2.2 is in VLAN 2.
  
  

Figure 5-3: Example of Routing Between Native and Non-Native 802.1Q VLANs

To configure VLANs for routing using 802.1Q VLAN encapsulation as shown in Figure 5-3, use the following steps beginning in global configuration mode.

Monitoring and Verifying VLAN Operation

Once the VLANs are configured on the switch router, you can monitor their operation using the following command.

To configure encapsulation over the EtherChannel, see the "About Encapsulation over EtherChannel" section.