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| 4.1 |  | Methods of Inter-VLAN Routing |
| 4.1.3 |  | Inter-VLAN routing performance and scalability issues |

For networks with just a couple of VLANs, using a physical interface for each VLAN may be a viable Inter-VLAN routing strategy. However, for networks with more than a few VLANs, a routing strategy that uses virtual interfaces becomes a must. Both an external and an internal router can be used with subinterfaces. The difference between the two solutions involves a tradeoff between cost and performance. An entry level external router is generally cheaper than a switch with layer 3 functionality, but the lack of integration between the layer 2 and 3 elements of the network and the “up and back” nature of external routers will limit performance. Once the 100 Mbps capacity of the external routers link is exceeded the use of an integrated route processor becomes the only realistic option for providing Inter-VLAN routing.

Regardless of the routing technique chosen, it is essential that the switched elements of the access and distribution layer be aligned with the routing functionality at the distribution layer. In particular, care should be taken to ensure that the distribution switch is the root of the spanning tree for each VLAN that is routed by the distribution router that is likely to be all VLANs. This step is easily overlooked and although a network will provide connectivity with an access layer switch acting as the STP root, the inefficient traffic flow will seriously limit the throughput of the distribution layer links.