Make life easier on campus with a simpler LAN
Three layers into one will go
The campus LAN is probably the most common network in use today, but its customary trio of layers is coming under examination as the need to reduce costs, add wireless access and increase performance continues to grow.
Back to basics: a campus LAN interconnects users in separate and multi-floored buildings in a smallish area, such as a university campus.
The numbers involved can be large – upwards of 25,000 registered devices with 2,000 to 3,000 concurrent users who may be using desktop PCs, cabled or Wi-Fi notebooks, or Wi-Fi or 3G-connected tablet computers and smartphones.
Then there may be voice over IP phones and radio frequency identification readers and other security devices.
The users want to be able to gain access to the building, send email, use word processing and spreadsheets, run server applications across the campus in a data centre, access web sites and use social media.
Trouble at t’mill
Whatever they are doing now, more of them will be doing more of it in a few months’ time.
The campus network is usually divided into three layers: an access layer, then an aggregation (or distribution) layer and finally a core-switching layer. The rationale for this network design is to keep trouble in one part of the network, on a floor of a building for example, from affecting other parts.
Each layer has its own devices. The access layer has access switches to which groups of edge devices connect. Then groups of access switches connect to distribution devices and these in turn connect to one or more core switches.
Each floor in a building may have one or more access switches and the building probably has its own distribution switch.
These switches connect to a core switch, creating a three-layer cake of devices with data traffic from a user in one building to a server in another, for example, passing through the user's nearby access switch.
The data moves on to the building's distribution switch, then to the core switch where it is routed to the destination building's distribution switch, then to the access switch for the device and finally to the server. Five network devices are used.
If one layer of this network design could be removed, such as the distribution layer, then there would be three devices between the user and the server.
In a large campus LAN this could easily mean getting rid of a hundred or more devices.
One idea is to aggregate the core and distribution layers by linking core switches directly to access switches (HP supports this approach).
The core switch still handles the same number of packets of data, but instead of coming from, say, 50 distribution switches they come from 250 access switches.
Something old, something new
This means the expanded core switch requires 250 incoming ports instead of 50 and must be able to route traffic from any port to another.
A problem at the edge is that the growth in the the number of users and devices, and hence traffic growth, means that new switch ports must be added.
For example, wireless access added to a building could mean another 500 accessing devices needing more ports and thus more access switches.
It is a good idea to be able to simply stack access switches on top of one another, so to speak, and then to manage the collection of stacked switches from a central point as a single management domain.
It is a good idea to be able to mix the different kinds of switch technologies
Over time switch technology advances and you end up with older and newer switches. It is clearly a good idea to be able to mix the different kinds of switch technologies in a stack so you can continue to use the older switches as the newer ones arrive.
More intelligence in these access layer switches enables this to happen. In addition, aggregating the core and distribution layers can be useful. Brocade provides a unique way to do this.
For example, Brocade is developing HyperEdge technology to view and manage different switches as a single logical device.
HyperEdge also enables different types of switches to be stacked together, the newer switches sharing more advanced services with other members in the stack.
Campus networks tend to be expensive but their use is growing. Collapsing the network layers makes for a simpler wiring task, with fewer devices to purchase and a less onerous management responsibility.
The ability to mix and match different generations of access switches and manage them in a single logical management domain is another incremental improvement. It reduces the difficulties involved in growing the scale of a campus LAN.
A three-layer wired design, with separate per-layer management domains that works for 5,000 devices and 500 users, is unlikely to be cost effective for 50,000 devices and 5,000 users.
It would make sense to move to a two-layer architecture and have a resilient edge switch concept. This would enable you to use switches for as long as they are useful, while bringing in newer switches that help scale the numbers and types of devices at the network edge.
Whatever devices you buy or lease for the layers in your campus LAN, their architectures should provide an open road to scalable growth, enabling you to simplify operations and lower your total cost per port. ®
re. "Trouble at t'mill"
It's "Trouble at ' mill". The missing word 'the' is not pronounced in any part and the final 't' in 'at' has a definite vocal stop followed by a brief pause in place of the missing word 'the'.
I can't decide whether this is an advert for switch gear or just a bad idea. From all the networks I have ever seen, none of them have been designed with the mentaility of "lets see how many devices we can use for the sake of it." Also, when it talks about removing a layer such as the distribution layer, it fails to consider that it may be far more cost effective to keep it than to do what the article proposes.
Take the university where I studied, each student room comes equipped with an ethernet port. A hall of residence may have 20 blocks, each containing 12 rooms. The way the blocks are laid out means it's not feasible to run the access cabling between blocks, so as a result, each block has a small switch such as a Cisco 2960 with uplinks to a distribution switch in an office (say avg. 70m per link), and then finally a main trunk back to the core switch in the university's server room. The distance between the distribution switch and the core switch is on the order of 500m. If you removed the distribution layer of this network, suddenly the core switch would need to handle 20 times the number of ports than before, and you would need to run over 10km of fibre to achieve the what was previously done with under 2km.
In short, even if you're clever with your cabling, surely the cost of moving away from the three layer model becomes prohibitative once all the extra costs are factored in?
Re: Somone think of the ARP table!
You've eliminated the large broadcast domain by routing, but you've increased the size of the ARP table. You now need a device with larger ARP tables than you used to with 3 tiers, which current cheap silicon doesn't have, since it was built expecting to need only enough ARP entries for a 3 tier application. (And typically ARP tables are replicated across a stack, so they don't scale up).