EXPERT RESPONSE
Wow. I could write a whole book or a half dozen about LAN network design considerations. In fact there are five very good books sitting on my bookshelf. However, to make sure that you get useful information before you turn 90, I have included a few tips here that are considerations to make before you embark on your LAN design journey. So while I can't fully explain every facet of LAN design, here are some of my top considerations that are often overlooked.
 Tip 1: VLANS, more VLANS, and more VLANS … Oh my!
VLANS are your friends. They are there to help segment out traffic in LAN environments to prevent excessive multicast and broadcast traffic. However it's very easy to get carried away with IP addressing and VLAN management. In a 300 node network, three floors and a single server room you shouldn't require more than a few VLANS (if that). When I worked at a university, we had a VLAN for servers, students, professors, and the wireless network. While we probably should have sat down and allocated the right subnetting scheme for the exact number of anticipated hosts, we decided to keep a clean, consistent configuration that was easier for us to maintain. And you know what, it worked. So just remember K.I.S.S.: Keep it simple silly.
Tip 2: Spanning tree loops.
On the other end of the spectrum, I once walked into an environment where no VLANs were used in a 1,000 node flat network. Needless to say, they needed some assistance in finding out why they had bridging loops and broadcast storms were taking down the entire network during peak hours. In a typical LAN design you want to steer clear of bridging loops by reducing as many multiple paths (blocked ports) as possible. In general, pick a root bridge, document it, and understand where potential bridging loops might occur. This actually leads into my last tip.
Tip 3: Document your network and keep it maintained.
I will be the first to admit that this is my least favorite task in network design and implementation. However even the most inspired network design with its perfect, color-coded cabling needs to be designed with two things in mind: maintenance and troubleshooting. If you are wise and document as you go, you'll reduce hours and headaches when that midnight emergency network issue call comes in. And believe me, it will eventually happen.
Save yourself the trouble and document as you go. One of the best habits I made myself do was update the description statements on my interfaces every time there was a change. By documenting the network, if issues arise, you'll save yourself hours when it really counts. Keep up the good work and have fun on your LAN design journey.
For additional information, Cormac Long wrote a great article right here at SearchNetworking.com on LAN design.
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