Specifications

Configuring Routing Between VLANs
Information About Routing Between VLANs
9
strips the 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.
Implementing VLANS
Network managers can logically group 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.
Issues regarding creating VLANs should have been addressed when you developed your network design.
Issues to consider include the following:
• Scalability
• Performance improvements
• Security
• Network additions, moves, and changes
Communication Between VLANs
Cisco IOS software provides full-feature routing at Layer 3 and translation at Layer 2 between VLANs.
Five different protocols are available for routing between VLANs:
• Inter-Switch Link Protocol, page 9
• IEEE 802.10 Protocol, page 10
• IEEE 802.1Q Protocol, page 10
• ATM LANE Protocol, page 10
• ATM LANE Fast Simple Server Replication Protocol, page 10
All five of these technologies are based on OSI Layer 2 bridge multiplexing mechanisms.
Inter-Switch Link Protocol
The Inter-Switch Link (ISL) protocol is used to interconnect two VLAN-capable Ethernet, Fast Ethernet,
or Gigabit Ethernet devices, such as the Catalyst 3000 or 5000 switches and Cisco 7500 routers. The ISL
protocol is a packet-tagging protocol that contains a standard Ethernet frame and the VLAN information
associated with that frame. The packets on the ISL link contain a standard Ethernet, FDDI, or Token Ring
frame and the VLAN information associated with that frame. ISL is currently supported only over Fast
Ethernet links, but a single ISL link, or trunk, can carry different protocols from multiple VLANs.
Procedures for configuring ISL and Token Ring ISL (TRISL) features are provided in “Configuring
Routing Between VLANs with Inter-Switch Link Encapsulation” section on page 15.