WAN Design

For this section of the course I decided to sort of start from the outer edge of the enterprise, with WAN connections and Branch design and moving our way in towards the campus and datacenter.

Now over the years the WAN has changed a lot, mostly that now we have a lot of different options available. Each with heir own benefits and considerations for us as the network designers to take into account to ensure the most appropriate options are presented for the design.

Traditional WAN technologies fall into 3 categories. Circuit switched, packet switched, and leased lines. Circuit switched would be like the PSTN, which is the ‘public switched telephone network’, or ISDN, which is actually a communications standard for use over the PSTN. These are circuit switched because the whole circuit needed for the call to connect is determined by the system and is reserved for the duration of the call. This differs from a packet switched network where you have what’s called a virtual circuit. This is sometimes referenced as a Permanent virtual circuit, or a switched virtual circuit.

Each packet is routed or switched independently, which allows for redundant paths to the destination to exist and be used in the event of a failure. With PSTN if there’s a line failure your call will be disconnected and will need to be initiated again; in a packet switched network, when the next packet arrives, it can just be routed around the failure. 

It’s a virtual circuit because the actual physical path the data takes is generally not defined. The service provider just owns a lot of routers and lines connecting them and your data will take whatever the least cost path is based on whatever protocol is being used. In the case of MPLS this is typically multiprotocol BGP with label distribution protocol.

Finally we have leased lines. These are the ultimate in control. This is when the service provider has a line that it has laid, like fiber, that they aren’t using at the moment, so you have the ability to lease that line for your enterprise to exclusively use. This is seen a lot in connections from the campus to the datacenter or connecting 2 datacenters, where you need your 10, 20, 40Gb connections. Leased lines typically are very expensive, but may turn out to be worthwhile for the speed, or you also get this physical line, so you can get layer 2 connectivity across the line, often needed for some of your server high availability clustering features. Also, since your traffic is the only traffic going over these lines for the duration of the lease, this is about as secure as you can get when it comes to service provider offerings.

So those are the categories of connectivity technologies you’ll likely encounter not lets talk about the topologies available and some considerations for them.

hub and spoke

First up is the known and trusted hub and spoke topology. This is where you have a star formation; you know some main head office that’s your hub here and the branches or remote offices all connect in to the hub. This has some big benefits that rightfully makes it one of the most popular topologies I’ve seen. Number 2 here is almost entirely due to number 1, and that’s this topology requires the fewest number of links to allow for full connectivity. Fewer WAN links means lest recurring cost. This also means more simplified configuration when a new office is brought online, and simpler troubleshooting when there’s issues. Something some business go for as well is to have all of their branch’s internet traffic go through the hub firewall as well, allowing for centralized filtering and scanning of that traffic. You can imagine that might make your life easier, right? To have 1 spot to manage for these functions and features. 

In that same fashion you can also imagine the drawbacks I’m sure. You’ll need a pretty beefy router or firewall here at the hub to make sure it doesn’t get bogged down since all spoke to spoke traffic also needs to pass through the hub. Also this potentially provides a single point of failure at the hub, I’d surely hope that you put redundant devices at the hub to protect against device failure, but this still provides a single location where if the poorly placed backhoe comes and *schwooop* cuts that line then your spokes would have no connectivity to at least corporate resources and potentially internet access.

full mesh

So lets move over to the other extreme, the full mesh topology. This is of course the ultimate in redundancy. Assuming you have dynamic routing happening with OSPF or EIGRP or something, then if a path goes down, you can simply route around it. Say any one guy gets cut, *schwooop*, then you can just come over here through this guy to get to the destination. Similarly you have connectivity directly between all locations so there’s no one router that’s getting bogged down here trying to handle all of the traffic.

Note for test takers, you can probably feel it as a question, right? Like ‘How many links are required to connect 8 branch offices in a full mesh topology?’ This here is how you calculate the number of links required for a full mesh.

# links = (N – 1) * N / 2

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