Sometimes when engineers think of a branch office they think of a small 10 person remote office, but really the branch is any location where employees work that is geographically separate from your main headquarters. A branch can be huge, it could be as large as your main campus; it might also have very specific connectivity requirements for the network based around what functions that branch office serves for the company.
Before we get too far into the design recommendations and considerations for the branch office, lets just make sure we have this in perspective. Recall the enterprise architecture model with the 4 functional areas and submodules within them. The branch is intended to be a module in the remote area. The reason I bring this up is your company’s growth will be far easier if your designs are repeatable. If you have a cookie cutter design for your branch office you can just tweak for the specific needs and roll out, not only will that make planning and installing a new branch office far easier, but management and troubleshooting time with be significantly reduced. Documentation will be far easier, I mean you get my point, right? Of course each deployment will have its own requirements that may require some tweaks to the design, but overall it’s great to come up with and reuse the same design rather than reinvent the wheel every time.
Right, so when designing a branch office there’s some main considerations you’ll want to take into account. How much scalability is expected? This branch office might only have 25 people in it now, but does management expect to lease the suite next door and move 250 people into it?
What level of redundancy is required? Another way to put this is, how critical is this branch office to the company’s operations? This branch may be a design office where most of the work done is local on their machines with only the occasional file transfer to the file servers a few times a week. On the other hand, this branch office may be where an executive works that accesses a critical LOB application near constantly. This would certainly change the conversation a bit when talking about redundancy options for the design.
Will wireless services be needed, and to what extent? We’ll get into the details of this a bit later in the series when we cover wireless design.
Finally of course we need to know what kind of budget we have for the design.
So when you start thinking about a design for the branch, there’s a few physical components that are pretty much the building blocks. You see routers for WAN and internet access. Cisco recommends the ISR for its ability to provide a rich feature set in a single appliance. Over here we have a scale showing the models of ISR generation 2, aka ISR G2, routers and where they fit speedwise. It really just gives you a sense that the 891 is a small router that really might be given to a teleworker or used in a small office; whereas the 3945 is a big beefy router able to support more users and applications that require more bandwidth.
Depending on the size of the branch, you might be able to get by with an etherswitch module for an ISR which provides network switching capability with up to 48 ports, or you may want an external switch. The 3850 down here is great as a stackable switch, in a larger branch you might go for a chassis switch like the 4500 series in the middle here.
You may not always need it and in 2 of the 3 branch designs cisco recommends we don’t see them, but you may use a security appliance like the Cisco ASA firewall. A 5506 X is what’s on the right here.
Remember the design is based around what the business needs are at the branch. Most offices including branch offices have wireless capabilities, so you may have some access points, and depending on the extent of the wireless services required you may even have a controller at the branch, perhaps the WLC module for the ISR.
Similarly for IP phones. Most businesses now use IP phone systems and you may have only a centralized call processing system in your server farm or datacenter, or also have minimal call processing capabilities at the branch in case the MPLS goes down that the phones are still functional. We’ll go into this a bit more later in the series.