6.1 VLAN Issues
6.1.5 Supporting multiple VLAN traffic
A VLAN is a logical broadcast domain. According to the rules of routing and bridging defined by the Open System Interconnection (OSI) reference model, to allow data to traverse between two VLANs, a Layer 3 device must be used to route between the VLANs. Either an internal or external Route Processor can be used to achieve this routing process.

As the number of VLANs increases in a network, network administrators must determine whether they want to have an individual router interface per VLAN , or if they want to use VLAN trunking to assign multiple VLANs to a single router interface .

One solution is to dedicate one interface on the Route Processor for each VLAN supported. However, as the number of VLANs per switch increases, so does the requirement for the number of interfaces on the Route Processor. In addition, some VLANs may not require inter-VLAN routing on a regular basis, creating a situation where interfaces on the Route Processor are underutilized.

Another solution is to carry multiple VLAN traffic over a single link. In order to maintain integrity between VLAN traffic, a mechanism is required to identify the packet of each VLAN. The ISL protocol is used to interconnect two VLAN-capable Fast Ethernet devices such as a router and a switch. The ISL protocol is a proprietary frame-tagging protocol that contains a standard Ethernet frame and the VLAN information associated with that frame.

ISL is currently supported over Fast Ethernet links, but a single ISL link, or trunk, can carry traffic from multiple VLANs. The IEEE 802.1Q protocol is also used to interconnect VLANs. This is a standards-based protocol that is supported between multiple vendors.

In Figure , the clients - on VLANs 10, 20, and 30 - all need to establish sessions with a server that is attached to a port in VLAN 60. Because the file server resides in a different VLAN than any of the requestors, you need to configure inter-VLAN routing. The Route Processor would perform this function as follows:

  1. The Route Processor accepts the packets from each VLAN because the Route Processor is configured to route VLAN 10, 20, and 30 traffic as well as VLAN 60 traffic to the server.
  2. The Route Processor then classifies the packet based on the destination network address. The Route Processor prepends the packet with an ISL VLAN header, in this case ISL = 60, appropriate to the destination subnet. The prepending action is what differentiates this as a routing process rather than a switching process.
  3. The router then routes the packets to the appropriate interface, in this case to FastEthernet 3/1.