Content

| 2.1 |  | VLAN Basics |
| 2.1.6 |  | VLANs versus router latency |

In a traditional routed network, packets going between networks have to be routed by a Layer 3 router. Every router that handles the data increases the end-to-end latency. Latency constitutes the amount of time necessary to transport a frame from the source port to the destination port. The latency could be reduced by replacing the routers with Layer 2 switches, but the network would then become a single broadcast domain and lose other advantages of Layer 3 segmentation such as security management and traffic filtering. VLANs have the advantages of both the decreased latency of Layer 2 switching, and the segmentation of Layer 3 routing.

Campus-wide VLANs were once the answer to Layer 3 latency, but they introduced their own complexities into the network. Campus-wide VLANs are still appropriate in some circumstances, however Cisco recommends designing networks using the Local VLAN model. The Local VLAN model has VLANs dedicated to each access layer switch cluster, with Layer 3 switching between wiring closets performed at the distribution layer or Main Distribution Facility (MDF). With both the Local VLAN and Campus-wide VLAN model, VLANs have many advantages over traditional switched networks, including segmentation of broadcast domains.

In modern campus operations, Layer 3 latency issues become rare as a result of Layer 3 switching or multilayer switching (MLS). MLS is basically hardware-based routing. In Cisco switches that support MLS, the first packet is routed in software by the router, with subsequent packets handled in hardware by the switch. On some Cisco switches, Cisco Express Forwarding (CEF) can be used. CEF uses a method of communicating information between the router and the switches which allows packets to be forwarded by the switches without ever needing to go to the router. Both MLS and CEF are discussed in more detail later.