Specifications

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Cisco 1751 Router Software Configuration Guide
OL-1070-01
strips header and forwards the frame to interfaces that match the VLAN color. If you are using a Cisco
network management product such as VlanDirector, you can actually color code the VLANs and
monitor VLAN graphically.
Why Implement VLANs?
Network managers can group logically networks that span all major topologies, including high-speed
technologies such as, ATM, FDDI, and Fast Ethernet. By creating virtual LANs, system and network
administrators can control traffic patterns and react quickly to relocations and keep up with constant
changes in the network due to moving requirements and node relocation just by changing the VLAN
member list in the router configuration. They can add, remove, or move devices or make other changes
to network configuration using software to make the changes.
Benefits and drawbacks of creating VLANs should be considered when you design your network,
including these issues:
• Scalability
• Performance improvements
• Security
• Network additions, moves, and changes
Communicating Between VLANs
The Cisco 1751 router uses the IEEE 802.1Q protocol for routing between VLANs.
The IEEE 802.1Q protocol is used to interconnect multiple switches and routers and for defining VLAN
topologies. IEEE 802.1Q support is currently available only for Fast Ethernet interfaces.
Procedures for configuring routing between VLANs with IEEE 802.1Q encapsulation are provided in
the “Configuring Routing Between VLANs with IEEE 802.1Q Encapsulation” chapter later in this
publication.
VLAN Translation
VLAN translation refers to the ability of the Cisco IOS software to translate between different virtual
LANs or between VLAN and non-VLAN encapsulating interfaces at Layer 2. Translation is typically
used for selective inter-VLAN switching of non-routable protocols and to extend a single VLAN
topology across hybrid switching environments. It is also possible to bridge VLANs on the main
interface; the VLAN encapsulating header is preserved. Topology changes in one VLAN domain do not
affect a different VLAN.
Designing Switched VLANs
By the time you are ready to configure routing between VLANs, you will have already defined them
through the switches in your network. Issues related to network design and VLAN definition should be
addressed during your network design. Refer to the Cisco Internetworking Design Guide and
appropriate switch documentation for information on these topics: