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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:
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.
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:
To configure the ISL VLANs shown in Figure 5-2, use the following procedure beginning in global configuration mode.
| Step | Command | Purpose | ||
|---|---|---|---|---|
| | Enters subinterface configuration mode to configure the Fast Ethernet interface f2.1. | ||
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| | Assigns VLAN 50 to bridge group 1. When you are configuring VLAN routing, skip this step. | ||
| | Enters Ethernet interface configuration mode to configure the Fast Ethernet interface f1. | ||
| | Assigns interface f1 to a bridge group. | ||
| | Returns to global configuration mode. | ||
| | Specifies that bridge group 1 will use the IEEE Ethernet Spanning-Tree Protocol. | ||
| | Enters interface configuration mode to configure the Fast Ethernet subinterface f2.2 | ||
| | Uses ISL to encapsulate the Ethernet frames sent from subinterface f2.2 with a header that maintains VLAN ID 100 between network nodes. | ||
| | Assigns VLAN 100 to bridge group 2. | ||
| | Enters interface configuration mode to configure the Fast Ethernet interface f3. | ||
| | Assigns interface f3 to bridge group 2. | ||
| | Returns to global configuration mode. | ||
| | Specifies that bridge group 2 will use the IEEE Ethernet Spanning-Tree Protocol. | ||
| | Returns to privileged EXEC mode. | ||
| | Saves your configuration changes to NVRAM. |
To monitor the VLANs once they are configured, refer to the "Monitoring and Verifying VLAN Operation" section.
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:
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.
| Step | Command | Purpose | ||
|---|---|---|---|---|
| | Enters subinterface configuration mode to configure the Fast Ethernet subinterface f2.1. | ||
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| Uses 802.1Q to send the Ethernet frames from subinterface f2.1 to VLAN 1 without any encapsulation. | ||
| | Configures an IP addresses on the subinterface. | ||
| | Enters interface configuration mode to configure the Fast Ethernet subinterface f2.2. | ||
| | Uses 802.1Q to encapsulate the Ethernet frames sent on subinterface f2.2 for VLAN 2. | ||
| | Configures an IP addresses on the subinterface. | ||
| | Returns to privileged EXEC mode. | ||
| | Saves your configuration changes to NVRAM. |
| Command | Purpose |
|---|---|
show vlan vlan-id | Displays information on all configured VLANs or on a specific VLAN (by VLAN ID number). |
To configure encapsulation over the EtherChannel, see the "About Encapsulation over EtherChannel" section.
Posted: Wed May 3 14:41:38 PDT 2000
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