Administration Guide at Cisco Unified CallManager Administration Guide, Release 5.0(4)
Content 2.1 Implementing Best Practices for VLAN Topologies 2.1.6 Describing End-to-End VLANs The term end-to-end VLAN refers to a single VLAN associated with switch ports that are widely dispersed throughout an enterprise network. Traffic for the VLAN is carried throughout the switched network. If many VLANs in a network are end-to-end, special links (trunks) are required between switches to carry the traffic of all the different VLANs. An end-to-end VLAN has these characteristics: Because a VLAN represents a Layer 3 segment, end-to-end VLANs allow a single Layer 3 segment to be geographically dispersed throughout the network. Reasons for implementing this design might include the following: Some items should be considered when implementing end-to-end VLANS. Switch ports are provisioned for each user and associated with a given VLAN. Because users on an end-to-end VLAN may be anywhere in the network, all switches must be aware of that VLAN. This means that all switches carrying traffic for end-to-end VLANs are required to have identical VLAN databases. Also, flooded traffic for the VLAN is, by default, passed to every switch even if it does not currently have any active ports in the particular end-to-end VLAN. Finally, troubleshooting devices on a campus with end-to-end VLANs can be challenging, because the traffic for a single VLAN can traverse multiple switches in a large area of the campus. For example, in a military setting, one VLAN is designated to carry top-secret data. Users with access to that data are widely dispersed throughout the network. Because all devices on that VLAN have similar security requirements, security is handled by access lists at the Layer 3 devices that route traffic onto the segment (VLAN). Security can be applied VLAN-wide without addressing security at each switch in the network, which might have only a single user on the top-secret VLAN.
Content 2.1 Implementing Best Practices for VLAN Topologies 2.1.7 Describing Local VLANs In the past, network designers attempted to implement the 80/20 rule when designing networks. The rule was based on the observation that, in general, 80 percent of the traffic on a network segment was passed between local devices, and only 20 percent of the traffic was destined for remote network segments. Therefore, end-to-end VLANs were typically used. Designers now consolidate servers in central locations on the network and provide access to external resources such as the Internet through one or two paths on the network, since the bulk of traffic now traverses a number of segments. Therefore, the paradigm now is closer to a 20/80 proportion in which the greater flow of traffic leaves the local segment, so local VLANs have become more useful. Additionally, the concept of end-to-end VLANs was very attractive when IP address configuration was a manually administered and burdensome process. Therefore, anything that reduced this burden as users moved between networks was an improvement. But, given the ubiquity of DHCP, the process of configuring IP at each desktop is no longer a significant issue. As a result, there are few benefits to extending a VLAN throughout an enterprise. It is often more efficient to group all users of a set of geographically common switches into a single VLAN, regardless of the organizational function of those users, especially from a troubleshooting perspective. VLANs that have boundaries based upon campus geography rather than organizational function are called “local VLANs.” Local VLANs are generally confined to a wiring closet. Here are some local VLAN characteristics and user guidelines: VLANs on a given access switch should not be advertised to all other switches in the network.
Content 2.1 Implementing Best Practices for VLAN Topologies 2.1.8 Benefits of Local VLANs in Enterprise Campus Network Local VLANs are part of the ECNM design, where VLANs used at the access layer should extend no further than their associated distribution switch. Traffic is routed from the local VLAN as it is passed from the distribution layer into the core. This design can mitigate Layer 2 troubleshooting issues that occur when a single VLAN traverses the switches throughout an enterprise campus network. Implementing the ECNM using local VLANs provides the following benefits:
Content 2.1 Implementing Best Practices for VLAN Topologies 2.1.9 Mapping VLANs in a Hierarchical Network When mapping VLANs onto the new hierarchical network design, keep these parameters in mind.