Network Components and Reading a Topology Diagram

What it is

Network components are the devices that connect, direct, filter and balance traffic, and a topology diagram shows how they are joined.

Switches connect devices in a network, routers connect networks, firewalls filter traffic and load balancers share it across servers. Other devices include access points and proxies.

On the exam you have to name the role of each device and read a diagram to trace a path or find a fault.

How it works

Each device works at a specific layer and has one main job.

Common components

Switch

A switch connects end devices in one network and learns MAC addresses. Each port is a separate collision domain, and all ports in a VLAN share one broadcast domain.

Router

A router joins different networks. It reads the destination IP address, finds the best route and sends the packet out of the right interface. Each router interface sits in a different subnet.

Firewall

A firewall sits between zones, such as inside and outside, and applies rules. A next generation firewall also inspects applications and detects threats. Rules usually say allow or deny for a source, destination and port.

Load balancer

A load balancer presents one address, called a virtual IP, and sends each request to one of several servers. It checks the health of the servers and stops using one that fails.

Reading a topology diagram

A diagram uses symbols for devices and lines for links. Labels give names, addresses, VLANs and port numbers.

                  Internet
                     |
                  [FW1]  Gi0/0 outside
                     |  Gi0/1 inside 10.0.0.1/24
                  [R1]   Gi0/2 trunk
                     |
                  [SW1]  VLAN 10 and VLAN 20
                  /      \
         Gi0/1  /          \  Gi0/2
          [Web1]            [DB1]
         VLAN 10           VLAN 20

Here are the facts that you can read from it:

• Traffic from the internet reaches the firewall first, then the router, then the switch.
• The firewall has an outside and an inside interface, and it controls what enters the network.
• The router and switch are joined by a trunk, which carries both VLANs.
• Web1 is in VLAN 10 and DB1 is in VLAN 20, so traffic between them must be routed, and a firewall or router rule can control it.
• Each device has labelled ports, which tell you where to look for a fault.

Port values on a diagram

Diagrams may also show transport ports on the links or rules, such as TCP 443 to the web server and TCP 3306 to the database. These tell you what traffic should be allowed along each path.

Tracing a path

1. Find the source and the destination.
2. Follow the links device by device.
3. At each device, ask what it does: switch inside the VLAN, route between subnets, filter, or balance.
4. Note any VLAN, address or port label that changes along the way.

Using a diagram to find a fault

If Web1 cannot reach DB1, look at the points on the path. The switch ports must be in the right VLANs.

The router needs a route or an interface for each VLAN. A rule on the router or firewall may block the database port.

Questions may add details such as a duplicated address, a port in the wrong VLAN or a missing route. Match the symptom to the device that is responsible.

Common symbols

Most diagrams show a router as a circle with arrows, a switch as a box with arrows, a firewall as a brick wall or flame, and a cloud for the internet or a network that you do not manage. A load balancer is often shown as a box with arrows that fan out.