Routing with Multiple Physical Links

The simplest and most straightforward method of routing between VLANs is to use several physical links between switches and an external router. Each link is configured for a single VLAN, so that a link is there for each VLAN to be routed. This approach is shown in Figure 7-2. Catalyst A is configured for three VLANs (1, 2, and 3). The switch is connected to the router using three separate links, each assigned to one of the three VLANs. Catalysts B and C each have only one VLAN (4 and 5, respectively). Each requires only a single link to the router. Therefore, five VLANs exist on this network and are interconnected by a single router with five network links.

Figure 7-2 InterVLAN Routing Using Multiple Physical Links VLAN 1

VLAN 5

Figure 7-2 InterVLAN Routing Using Multiple Physical Links VLAN 1

VLAN 5

To illustrate the routing operation, suppose a station connected to VLAN 1 on Catalyst A needs to communicate with a station connected to VLAN 5 on Catalyst B. Because the destination has an IP address that is off the local network, the source station will send the packets to the router. These packets will be sent out the VLAN 1 link on Catalyst A. The packets will be examined by the router and sent over the VLAN 5 link to Catalyst B. In this fashion, every packet travels across a link reserved for traffic on a specific VLAN and passes through the router.

Using one VLAN per link offers an intuitive approach to routing between VLANs. Routers naturally associate each physical link with a subnetwork (some Layer 3 protocol and address range), and transport packets between links. Each link is also inherently segmented from the others, unless bridging arrangements are made within the router. This method is useful when the switches and router are already available and can be quickly connected, using a small number of VLANs. No special configuration is needed, other than the usual interface addressing used on the router.

As the network grows, this method can quickly get out of hand. Every additional VLAN requires an additional physical link to the router. Clearly, the expense and logistics of adding a large number of connections to the router outweigh the simplicity of the design.

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