networks. Although the Internet serves as the obvious example, this point is true for any type of network, such as a large campus backbone. Because routers prevent broadcast propagation and use more intelligent forwarding algorithms than bridges and switches, routers provide more efficient use of bandwidth. This simultaneously results in flexible and optimal path selection. For example, it is very easy to implement load balancing across multiple paths in most networks when routing. On the other hand, Layer 2 load balancing can be very difficult to design, implement, and maintain. If a VLAN spans across multiple devices a trunk is used to interconnect the devices. A trunk carries traffic for multiple VLANs. For example, a trunk can connect a switch to another switch, a switch to the inter-VLAN router, or a switch to a server with a special NIC installed that supports trunking. Remember that when a host on one VLAN wants to communicate with a host on another, a router must be involved. Interactive Media Activity Drag and Drop: Inter-VLAN Routing When the student has completed this activity, the student will learn the path packets take in a network with inter-VLAN routing. The student will predict the path a packet will take given the source host and the destination host.
Content 9.3 Inter-VLAN Routing Overview 9.3.3 Inter-VLAN issues and solutions When VLANs are connected together, several technical issues will arise. Two of the most common issues that arise in a multiple-VLAN environment are: When a device needs to make a connection to a remote host, it checks its routing table to determine if a known path exists. If the remote host falls into a subnet that it knows how to reach, then the system checks to see if it can connect along that interface. If all known paths fail, the system has one last option, the default route. This route is a special type of gateway route, and it is usually the only one present in the system. On a router, an asterisk (*) indicates a default route in the output of the show ip route command. For hosts on a local area network, this gateway is set to whatever machine has a direct connection to the outside world, and it is the Default Gateway listed in the workstation TCP/IP settings. If the default route is being configured for a router which itself is functioning as the gateway to the public Internet, then the default route will point to the gateway machine at an Internet service provider (ISP) site. Default routes are implemented using the ip route command. Router(Config)#ip route 0.0.0.0 0.0.0.0 192.168.1.1 In this example, 192.168.1.1 is the gateway. Inter-VLAN connectivity can be achieved through either logical or physical connectivity. Logical connectivity involves a single connection, or trunk, from the switch to the router. That trunk can support multiple VLANs. This topology is called a router on a stick because there is a single connection to the router. However, there are multiple logical connections between the router and the switch. Physical connectivity involves a separate physical connection for each VLAN. This means a separate physical interface for each VLAN. Early VLAN designs relied on external routers connected to VLAN-capable switches. In this approach, traditional routers are connected via one or more links to a switched network. The router-on-a-stick designs employ a single trunk link that connects the router to the rest of the campus network. Inter-VLAN traffic must cross the Layer 2 backbone to reach the router where it can move between VLANs. Traffic then travels back to the desired end station using normal Layer 2 forwarding. This out-to-the-router-and-back flow is characteristic of router-on-a-stick designs. Interactive Media Activity Drag and Drop: Inter-VLAN Routing Issues and Solutions When the student has completed this activity, the student will learn about some of the problems when using VLAN. They will also learn some of the solutions.
Content 9.3 Inter-VLAN Routing Overview 9.3.4 Physical and logical interfaces In a traditional situation, a network with four VLANs would require four physical connections between the switch and the external router. As technologies such as Inter-Switch Link (ISL) became more common, network designers began to use trunk links to connect routers to switches. Although any trunking technology such as ISL, 802.1Q, 802.10, or LAN emulation (LANE) can be used, Ethernet-based approaches such as ISL and 802.1Q are most common. The Cisco Proprietary protocol ISL as well as the IEEE multivendor standard 802.1q are used to trunk VLANs over Fast Ethernet links. The solid line in the example refers to the single physical link between the Catalyst Switch and the router. This is the physical interface that connects the router to the switch. As the number of VLANs increases on a network, the physical approach of having one router interface per VLAN quickly becomes unscalable. Networks with many VLANs must use VLAN trunking to assign multiple VLANs to a single router interface. The dashed lines in the example refer to the multiple logical links running over this physical link using subinterfaces. The router can support many logical interfaces on individual physical links. For example, the Fast Ethernet interface FastEthernet 0/0 might support three virtual interfaces numbered FastEthernet 1/0.1, 1/0.2 and 1/0.3. The primary advantage of using a trunk link is a reduction in the number of router and switch ports used. Not only can this save money, it can also reduce configuration complexity. Consequently, the trunk-connected router approach can scale to a much larger number of VLANs than a one-link-per-VLAN design.
Content 9.3 Inter-VLAN Routing Overview 9.3.5 Dividing physical interfaces into subinterfaces A subinterface is a logical interface within a physical interface, such as the Fast Ethernet interface on a router. Multiple subinterfaces can exist on a single physical interface. Each subinterface supports one VLAN, and is assigned one IP address. In order for multiple devices on the same VLAN to communicate, the IP addresses of all meshed subinterfaces must be on the same network or subnetwork. For example, if subinterface 2 has an IP address of 192.168.1.1 then 192.168.1.2, 192.168.1.3, and 192.1.1.4 are the IP addresses of devices attached to subinterface 2. In order to route between VLANs with subinterfaces, a subinterface must be created for each VLAN. The next section discusses the commands necessary to create subinterfaces and apply a trunking protocol and an IP address to each subinterface.
Content 9.3 Inter-VLAN Routing Overview 9.3.6 Configuring inter-VLAN routing This section demonstrates the commands necessary to configure inter-VLAN routing between a router and a switch. Before any of these commands are implemented, each router and switch should be checked to see which VLAN encapsulations they support. Catalyst 2950 switches have supported 802.1q trunking since the release of Cisco IOS release 12.0(5.2)WC(1), but they do not support Inter-Switch Link (ISL) trunking. In order for inter-VLAN routing to work properly, all of the routers and switches involved must support the same encapsulation. On a router, an interface can be logically divided into multiple, virtual subinterfaces. Subinterfaces provide a flexible solution for routing multiple data streams through a single physical interface. To define subinterfaces on a physical interface, perform the following tasks: To identify the interface, use the interface command in global configuration mode. Router(config)#interface fastethernet port-number. subinterface-number The port-number identifies the physical interface, and the