Network Devices and Topologies

Network Devices and Their Forwarding Behavior

Understanding how each network device handles data is the foundation of troubleshooting and design. Four devices appear in nearly every modern network: the hub, the switch, the router, and the wireless access point (WAP). Each one makes forwarding decisions at a different layer of the OSI model and creates different traffic patterns on the wire.

Hubs

A hub operates at OSI Layer 1 (Physical). It has no intelligence: when a frame arrives on any port, the hub immediately repeats that electrical signal out of every other port. Every device connected to a hub shares the same collision domain and the same broadcast domain. This means only one device can transmit at a time without causing a collision, and every frame reaches every host whether it was intended for that host or not. Hubs are largely obsolete in modern networks but are still referenced on certification exams because they illustrate why switches were developed.

Switches

A switch operates at OSI Layer 2 (Data Link). It builds and maintains a MAC address table (also called a CAM table) by reading the source MAC address of every incoming frame and recording which port that address arrived on. When a frame arrives for a known destination MAC, the switch forwards it only out the port where that destination lives—not to every port. This process is called unicast forwarding.

When a switch receives a frame with an unknown destination MAC, or a broadcast frame (destination FF:FF:FF:FF:FF:FF), it floods the frame out all ports except the one it arrived on. Each port on a managed switch is its own collision domain, but all ports on the same VLAN share one broadcast domain.

You can view the MAC address table on a Cisco switch with the privileged-exec command:

```

switch# show mac address-table

```

The output lists the VLAN, MAC address, type (dynamic or static), and the port it was learned on.

Routers

A router operates at OSI Layer 3 (Network). It forwards packets based on destination IP addresses using a routing table, not MAC addresses. Crucially, a router is a broadcast domain boundary: broadcast traffic does not cross a router interface. Every interface on a router is its own broadcast domain, which is why routers are used to segment large networks and connect them to the internet.

Routers use routing protocols (such as OSPF or EIGRP) or static routes to populate their routing tables and determine the best path for each packet.

Wireless Access Points

A WAP connects wireless clients to a wired network at OSI Layer 2. It translates between the 802.11 wireless standard and the 802.3 wired Ethernet standard. From the network's perspective, a WAP acts similarly to a switch port: wireless clients receive their own Layer 2 segment. However, all clients on the same WAP and SSID share the same radio frequency, so wireless still involves a shared medium in the radio domain. A WAP does not break up broadcast domains by itself.

Physical vs. Logical Topologies

A physical topology describes how cables and devices are physically connected. A logical topology describes how data actually flows through the network, which may differ from the physical layout.

Common Physical Topologies

• Star: All devices connect to a central device (switch or hub). This is the most common topology in modern LANs. A single cable failure affects only one device.
• Bus: All devices connect to a single shared cable called the backbone or segment. A break anywhere on the bus takes down the whole segment. Largely obsolete.
• Ring: Devices connect in a closed loop; data travels in one direction (or two in dual-ring designs). Token Ring and SONET use this concept.
• Mesh: Every device connects to every other device (full mesh), or at least to more than one device (partial mesh). Highly redundant; used in WANs and critical infrastructure.
• Point-to-Point: A direct link between exactly two devices—for example, a router WAN interface connected to a service provider.
• Hybrid: A combination of two or more topologies. Most real-world enterprise networks are hybrid, such as a star topology within each floor connected by a partial mesh between buildings.

Broadcast Domains and Collision Domains Summary

| Device | OSI Layer | Collision Domains | Broadcast Domains |

|--------|-----------|-------------------|-------------------|

| Hub | 1 | 1 (all ports share) | 1 |

| Switch | 2 | 1 per port | 1 per VLAN |

| Router | 3 | 1 per interface | 1 per interface |

Lab Safety Notes

Before working in the network lab, follow these practices:

• Electrical safety: Rack-mounted equipment uses AC power. Never reach into a powered rack without confirming equipment is properly grounded. Do not work on cables near exposed power strips with wet hands.
• Cable management: Keep cable pathways clear of walkways to prevent trip hazards. Secure patch cables with Velcro ties, not zip ties that cannot be removed without cutting.
• ESD prevention: When handling any network interface card or internal component outside its chassis, wear an anti-static wrist strap bonded to an unpainted metal surface. Network switches and routers already assembled in their chassis do not require ESD precautions for normal port connections.
• Equipment handling: Rack-mounted gear can be heavy. Use proper lift technique—bend at the knees—and use two people or a lift for heavy chassis routers.