>I've been going through the BCMSN course and I'm a bit baffled on how to do >something. There's the statement that: > >Because VLANs terminate at the distribution device, core links are not trunk >links and traffic is routed across the core. > >What I'm puzzled by is how to terminate a VLAN at the distribution layer. >What am I missing here? Well, like OSI, it's a model without absolute rules, and, like OSI, it's evolved to use sublayers. I often have multiple sublayers in distribution, which very well may physically have both VLANs and VLAN trunks. Don't know if it will help, but here's some partially relevant discussion from a draft chapter of my upcoming book "Building Service Provider Networks": One useful and popular model to describe enterprise network architecture was introduced by Cisco Systems. Any model, of course, is a guideline, and, as shown in Figure 2, this model has been used with both WAN and LAN cores. Figure 2: 3 layer model This model divides the network into three tiers: o Access: contains end users and local servers. It is possible to put centralized servers in an access tier, but, when doing so, it is usually best to put the individual servers of a local cluster into the access tiers. Load distribution to these servers is at the next tier. o Distribution: contains devices that transition between environments (e.g., LAN to WAN, building to campus, or different transmission technology). Often, the distribution tier is the place that requires the greatest intelligence for protocol conversion, buffering, etc. Another term entering usage for this function is "Edge." o Core: efficiently links sites of the infrastructure. May be a collapsed LAN backbone primarily of layer 2 and inter-VLAN devices, or may be a set of routers. One enterprise guideline is that layer 2 relays tend to have all their interfaces inside tiers, while layer 3 (i.e., routers) and higher layer (e.g., firewalls and proxies) tend to have interfaces between different tiers. This guideline is not terribly rigorous, as a speed-shifting switch between a workgroup and a building (or campus) core often logically straddles the top of the access tier and the bottom of the distribution tier. Large distribution networks will include multiple levels of concentration. When demand access is involved (e.g., dialup), it can be convenient to put end hosts and access routers in the access tier, dial-in servers at the bottom of the distribution tier, and concentrating routers inside the distribution tier. Large routers link regions to a core router or complex of routers. Another function that fits nicely in the distribution tier is that of firewalls or border routers providing connectivity outside the enterprise. See Figure 3. Figure 3: Three-Level Model Details In this figure, note that the central servers themselves are at the distribution tier, but that user connectivity to them comes through the core, and they have their own inter-server links at the access tier. Having isolated links and possibly specialized hosts, such as backup machines, for large servers can keep a great deal of traffic localized and avoid negative performance impact. This model works well for networks of medium size. Small networks may collapse certain of the tiers together, and very large networks become more like carrier networks. In the optimal use of this model, the customer access router is closest to the end hosts, customer core routers link campuses or sites, and distribution routers perform concentration and translation functions between access and core. External connectivity is generally a function of the distribution tier, although, if all otherwise unknown traffic defaults to a central external router, that router might be in the customer core. The model had limitations in large enterprise networks, where there may be multiple operational levels of local, regional, and national/international corporate backbones. One approach, shown in Figure 4, is to apply the model recursively, where the top level of one organizational level becomes the bottom level of another organizational level. Figure 4: Recursive 3 Layer The recursive approach really didn't work well, because each tier, and the devices that commonly straddle them, really have distinctive characteristics. An access device really does not share characteristics with a core device in a larger network. Another method was to create additional core layers for major geographic levels, such as national and intercontinental. Figure 5 shows the logical design I did for an international manufacturing company, which had relatively little communications among their regions, but all regions had significant communications with headquarters. It was reasonable to have all inter-region communication go through headquarters. Figure 5: Multilevel Enterprise Core--Centralized Organization In this figure, note that the headquarters users and central servers were treated as a virtual region, rather than putting them into the core. The core should only be used for communications and carefully selected network management devices, never for application servers. Not every enterprise has the same requirements. Figure 6 shows my logical design for a worldwide transportation company that had both extensive inter-region communications, plus an Internet connectivity requirements from each region. 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