The goal of this post is to share my experience in using Cisco equipment to teach computer networks.
Computer networks is one of the only course in the computer science curriculum that can involve actual lab work, instead of just doing programming, and I think it’s interesting to do it “for real” instead of using simulators, even if some good ones exist (like kathara).
We use Cisco equipment because we have access to it. It’s also likely it will be the equipment students will use on their actual job. However this equipment is not fit for teaching. There are many things that are done by default which are probably very good in a real environment, but that complicates teaching.
The goal is to explain the best configuration of Cisco switches and routers to make them easier to use in a teaching environment.
I want to stress that some of the defaults in the configuration would be very bad in practice, in a real environment.
This article is neither an endorsement nor a criticism of Cisco products.
Introduction
This article is for people that are already familiar with using real equipment (switches, routers) for a computer networking course, and have a basic understanding of how to configure a Cisco equipment.
It assumes that the problem of how to access the equipment at any given moment to configure it is already solved. In my lab, we use a terminal server for that.
Some of this information might relate to old and obsolete equipment
Switches
VLANs
Even if VLANs are not in your curriculum, it’s still important to know the fundamentals to understand how the switch should be configured. Weird things might happen even with one vlan.
The tl;dr is that every “normal” (access) port in a switch has a vlan number. Only ports with the same vlan number can communicate with each other. By default, all ports are on vlan 1.
If two switches are linked together by a cable, then ports with the same vlan numbers across the two switches can communicate with each other. For this to work, the ports that link the two switches together should be a trunk port.
CDP
Why to deactivate: Weirds things might happen if you connect two switches together, unnecessary traffic.
CDP is a proprietary cisco protocol for equipments to share info, like their name, operating systems, and some configuration parameters. The main reason to deactivate has to do with the configuration parameters. Every time the configuration of a switch changes, the CDP version number goes up. When two switches using the CDP protocol are connected together (I have no idea if the problem also appears with routers), the one with the least CDP version number will take its configuration from the one with the highest CDP version.