Real-world.

You don’t get a list of possible choices of why something is not working. Usually you get a ticket, or someone from Desktop Support or the Help Desk pings you saying (something along the lines of), “the Business Analysts are reporting the network is slow”.

What do you do.

First thing I like to verify – are ALL the analysts experiencing slow network speeds, or just some, or just 1 person?

Then ask more questions like…
    • When do you notice the slow speeds?
    • Is it only when working inside a shared drive?
    • Any slowness when working with your files locally on the machine?
    • Experiencing slowness when using a web app?
    • Do they notice slowness when working in the office or remotely?
Just one person.

Check to see if they experience slowness when logged onto another machine. Even test having them log on to a colleague’s machine who is not experience slowness.

All or some people.

I’ll want to check and see how they are connected to the network.

    • What switch?
    • What router?
    • What servers (onsite or offsite)?

Now what.

Let’s say you’re working for an awesome company who maintains updated diagrams of their full network. Awesome! Looking at the below diagram, you nail down that the Business Analysts in our scenario are connected to SwitchA, via a hub.

source:  Boson Ex-Sim

Next I would.

  • Log into SwitchA
  • Run the show interface fa0/1 command
  • Read through the output
  • Notice there’s a large number of collisions (in this particular example)
  • Wait a bit and re-run the command to see if the collision count is ticking up higher

Then.

  • Run the show run command
  • Scroll down to view what’s configured for fa0/1
  • Notice speed and duplex are manually configured and not set to auto
  • Look back at the diagram
  • Hunt for devices connected to Fa0/1 that may not run full-duplex

Bingo.

A hub.

They cannot run full-duplex, only half duplex. Devices connected to a hub cannot send & receive at the same time. It can only do one at a time. So I would –

  • Change Fa0/1 on SwitchA to half-duplex
  • Then wait a bit
  • Run the show interface fa0/1 command again
  • See if the collisions are ticking downwards
  • Then check with the Business Analysts after some time to see if speed has improved
GOOD READ: Common Causes of Slow IntraVLAN and InterVLAN Connectivity in Campus Switch Networks - Cisco.com