Connecting Devices (Hubs, Bridges, Switches, and Routers)

In computer networking, multiple independent network segments must be linked together to form a cohesive, scalable communication infrastructure. This is accomplished using specialized connecting devices that operate at different layers of the Open Systems Interconnection (OSI) model.

1. Five Categories of Connecting Devices

Connecting devices are categorized based on the highest OSI layer at which they operate. A device's functional layer dictates its ability to inspect, modify, and filter incoming traffic.

text

+-------------------------------------------------------------+
| Layer 5-7: Application/Presentation/Session |   Gateways    |
+-------------------------------------------------------------+
| Layer 3:   Network Layer                   | Routers/L3 Sw.|
+-------------------------------------------------------------+
| Layer 2:   Data Link Layer                 | Bridges/L2 Sw.|
+-------------------------------------------------------------+
| Layer 1:   Physical Layer                  | Hubs/Repeaters|
+-------------------------------------------------------------+

The table below outlines these devices alongside their active protocol layers and primary data units:

2. Physical Layer Devices: Repeaters and Hubs

Repeaters

A repeater is a Layer 1 device designed to extend the physical boundaries of a Local Area Network (LAN). Over long physical mediums, electrical, optical, or wireless signals suffer from attenuation, which limits the maximum segment length.

text

       +----------+                 +----------+
======| Segment 1 |=== [Repeater] ===| Segment 2 |======
       +----------+                 +----------+
      (Weakened Signal)            (Regenerated Signal)

Key characteristics of repeaters include:

No Filtering: A repeater has no logic to read MAC or IP addresses. It forwards every incoming bit, including collisions and corrupted packets.
Regeneration vs. Amplification: A repeater is a regenerator, not an amplifier. An amplifier simply boosts the analog signal, which also amplifies background noise and distortion. A repeater reconstructs the digital $1$ s and $0$ s bit-by-bit, removing noise and jitter before transmitting a clean, full-strength signal.

Hubs

A hub is essentially a multi-port repeater. It connects multiple hosts physically in a star topology but behaves logically as a shared bus topology.

Passive Hub: A passive hub acts merely as a physical connection point. It distributes incoming electrical signals to all other ports without regenerating them. It does not require external power.
Active Hub: An active hub regenerates the physical signal before broadcasting it to all other active ports. It requires external power.
Collision Domains: All ports on a hub belong to the same collision domain and the same broadcast domain. If two devices transmit simultaneously, a collision occurs.

3. Data Link Layer Devices: Bridges and Two-Layer Switches

Bridges

A bridge operates at Layer 2 (Data Link) and partitions a large, congested LAN into smaller, isolated collision domains.

text

             +-----------------+
             |     Bridge      |
             | [Filtering Table] |
             +--------+--------+
                      |
           +----------+----------+
           |                     |
     +-----+-----+         +-----+-----+
     |   LAN 1   |         |   LAN 2   |
     | (Segment) |         | (Segment) |
     +-----------+         +-----------+

No MAC Address Modification: A bridge reads the source and destination physical (MAC) addresses but does not alter them.
Collision Domain Isolation: Unlike a hub, a bridge isolates collision domains. If a collision occurs on LAN 1, it does not propagate to LAN 2. However, all connected segments still belong to the same broadcast domain.

Learning Bridges

A learning bridge dynamically builds and maintains its forwarding (or filtering) table. Initially, the table is empty. It populates this table by inspecting the source MAC address of incoming frames on each interface.

The learning process operates as follows:

When a frame arrives on Port $A$ $A$ with source MAC $S$ $S$ and destination MAC $D$ $D$ :
- The bridge logs $(S, A)$ in its lookup table.
The bridge checks if destination $D$ $D$ exists in the table:
- Case 1 (Unicast Flood): If $D$ is not in the table, the bridge floods the frame out of all ports except Port .

The Loop Problem and the Spanning Tree Protocol (STP)

To ensure high availability, network administrators often introduce redundant paths between switches or bridges. However, redundant Layer 2 topologies cause three severe issues if left unmanaged:

Broadcast Storms: Because Layer 2 Ethernet headers do not have a Time-to-Live (TTL) field, broadcast frames circulate indefinitely, saturating bandwidth and crashing switches.
MAC Table Instability: Switches continuously update their address tables as duplicate frames arrive from different ports, leading to constant database thrashing.
Multiple Frame Copies: Host systems receive duplicate copies of unicast frames.

text

        +------------+
   +---->  Switch A  <----+
   |    +-----+------+    |
   |          |           |
[Host 1]      |      [Host 2]
   |          v           |
   +---->  Switch B  <----+
        +------------+
    (Redundant loops generate broadcast storms)

To resolve this, bridges use the Spanning Tree Protocol (STP) (standardized in IEEE 802.1D). STPmodels the network as a graph $G = (V, E)$ , where $V$ represents the switches and LAN segments, and $E$ represents the links. It calculates a minimum spanning tree to systematically disable redundant links, leaving exactly one active logical path between any two nodes.

Spanning Tree Algorithm Steps:

Elect one Root Bridge: The bridge with the lowest Bridge ID (BID), which consists of a configurable Priority ( $2$ bytes) and the MAC address ( $6$ bytes).
Find Shortest Paths to the Root Bridge: Every non-root bridge calculates the path cost to the root. Let the cumulative link cost from bridge $B_i$ to the Root Bridge $R$ be: $Cost(B_i \to R) = \sum_{e \in \text{path}} Cost(e)$

4. Network Layer and Above: Routers, Three-Layer Switches, and Gateways

Routers

A router is a Layer 3 (Network) device that routes data packets across independent networks (different subnets, LANs, and WANs).

Addressing: Routers make decisions based on logical addresses (e.g., IPv4 or IPv6) rather than physical MAC addresses.
Broadcast Domains: Unlike bridges and switches, routers block Layer 2 broadcasts. They act as a boundary for broadcast domains.
Packet Modifications: When a router forwards a packet:
- The physical MAC source and destination addresses are stripped and rewritten at each hop to reflect the immediate local link.
- The logical IP source and destination addresses remain unchanged (except during Network Address Translation / NAT).

Three-Layer Switches

A three-layer (Layer 3) switch is a high-performance device that blends Layer 2 switching with Layer 3 routing capability.

Traditional routers perform routing in software using general-purpose CPUs.
Layer 3 switches use specialized hardware called Application-Specific Integrated Circuits (ASICs) to forward IP packets at wire speed.

Gateways

A gateway is a protocol converter that can operate across all seven layers of the OSI model. It translates data between completely incompatible network environments (for example, connecting a legacy IBM SNA mainframe architecture to an IP-based TCP/IP network).

5. Backbone Networks

A backbone network provides a high-capacity transmission path to link multiple individual LANs. No user stations are directly connected to the backbone; instead, the backbone connects network distribution devices (like switches and routers).

Bus Backbone

In a bus backbone, the underlying topology is a shared bus. A single transmission line (such as a coaxial cable or fiber run) links the different LAN distribution devices through bridges or switches.

text

   +-------+        +-------+        +-------+
   | LAN 1 |        | LAN 2 |        | LAN 3 |
   +---+---+        +---+---+        +---+---+
       | Bridge         | Bridge         | Bridge
=======#================#================#======= (Bus Backbone)

Star Backbone

In a star backbone, the backbone topology is a star. A central switch or router acts as the hub, connecting all individual LANs.

text

              +-------+
              | LAN 1 |
              +---+---+
                  |
            +-----+-----+
            | Central   |
  +---------+ Backbone  +---------+
  |         | Switch    |         |
+-+-+       +-----+-----+       +-+-+
|LAN 2|           |             |LAN 3|
+---+             |             +---+
              +---+---+
              | LAN 4 |
              +-------+

Remote LANs

Two geographicsally separated LANs can be linked using remote bridges connected over a point-to-point wide-area link (such as a leased fiber line or microwave link). The point-to-point link acts logically as a transparent bridge interface.

6. Virtual Local Area Networks (VLANs)

A Virtual Local Area Network (VLAN) is a logical local area network configured entirely through software on Layer 2 switches rather than physical wiring.

text

       +-----------------------------------------+
       |             L2 Switch                   |
       |  [VLAN 10: Sales]     [VLAN 20: IT]     |
       +----+---------+---------+---------+------+
            |         |         |         |
          +---+     +---+     +---+     +---+
          |PC1|     |PC2|     |PC3|     |PC4|
          +---+     +---+     +---+     +---+

Key Concepts of VLANs:

Broadcast Domains: Each VLAN acts as an isolated Layer 2 broadcast domain. Broadcast traffic from a device in VLAN 10 will not reach devices in VLAN 20, reducing unnecessary network overhead.
Security & Segmenting: Workgroups are isolated logically, preventing sensitive systems (like financial servers) from being visible to unauthorized network segments.
Inter-VLAN Communication: Because VLANs exist as separate subnets, devices in different VLANs cannot communicate with each other at Layer 2. To pass traffic between them, a Layer 3 device (such as a router or a Layer 3 switch) must route the packets.

Port Types in VLAN Configuration

Access Port: A switch port assigned to exactly one VLAN. End hosts (such as PCs and printers) are connected to access ports and remain unaware of any VLAN tagging.
Trunk Port: A link between two switches (or between a switch and a router) designed to carry traffic for multiple VLANs over a single physical cable. Frames sent over trunk links contain an explicit tag identifying their originating VLAN.

IEEE 802.1Q Frame Tagging

To identify which VLAN a frame belongs to as it crosses a trunk link, switches insert an 802.1Q tag into the standard Ethernet header. This $4$ -byte header insertion is placed directly after the Source MAC address field:

text

+----------+----------+-------------------+----------+---------+-----+
| Dest MAC | Src MAC  | 802.1Q Tag        | Type/Len | Payload | FCS |
| (6 bytes)| (6 bytes)| TPID/TCI (4 bytes)| (2 bytes)|         |     |
+----------+----------+-------------------+----------+---------+-----+

Within the $4$ -byte 802.1Q Tag:

Tag Protocol Identifier (TPID): Set to 0x8100 to denote that the frame contains 802.1Q tagging.
VLAN Identifier (VID): A $12$ -bit field specifying the VLAN ID (supporting up to $2^{12} - 2 = 4094$ distinct VLANs, excluding reserved values 0 and 4095).

7. Practical Configuration: VLANs and Inter-VLAN Routing

To understand how VLANs operate in practice, we will examine a step-by-step Cisco IOS configuration on a Cisco Switch and Router using Packet Tracer.

Scenario Topology

VLAN 10: Named SALES. IP Subnet: 192.168.1.0/24. Ports: fa0/1 and fa0/2.
VLAN 20: Named IT. IP Subnet: 192.168.2.0/24. Ports: fa0/3 and fa0/4.
Trunk Link: Port fa0/5 connected to the router's physical interface fa0/0.
Inter-VLAN Routing: Configured on the router using Router-on-a-Stick (R.O.A.S.) with logical sub-interfaces fa0/0.10 and fa0/0.20.

text

                          +------------+
                          |   Router   |
                          | (fa0/0)    |
                          +-----+------+
                                | (Trunk Link)
                                |
                          +-----+------+
                          | Switch     |
                          | (fa0/5)    |
                          +--+---+---+--+
                             |   |   |
         +-------------------+   |   +-------------------+
         |                       |                       |
   +-----+-----+           +-----+-----+           +-----+-----+
   |   VLAN 10 |           |   VLAN 10 |           |   VLAN 20 |
   |    PC1    |           |    PC2    |           |    PC3    |
   +-----------+           +-----------+           +-----------+

Step 1: Switch VLAN Creation

Create the two database entries for VLAN 10 and VLAN 20 on the switch:

text

Switch# configure terminal
Switch(config)# vlan 10
Switch(config-vlan)# name SALES
Switch(config-vlan)# exit
Switch(config)# vlan 20
Switch(config-vlan)# name IT
Switch(config-vlan)# exit

Step 2: Assign Interfaces to VLANs

Assign the switch access ports to their respective VLANs and configure fa0/5 as a trunk port to carry the multi-VLAN traffic up to the router:

text

! Configure ports fa0/1 and fa0/2 for VLAN 10 (SALES)
Switch(config)# interface fa0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10
Switch(config-if)# exit
 
Switch(config)# interface fa0/2
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10
Switch(config-if)# exit
 
! Configure ports fa0/3 and fa0/4 for VLAN 20 (IT)
Switch(config)# interface fa0/3
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 20
Switch(config-if)# exit
 
Switch(config)# interface fa0/4
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 20
Switch(config-if)# exit
 
! Alternative shortcut to configure ports in bulk:
! Switch(config)# interface range fa0/1 - 4
! Switch(config-if-range)# switchport mode access
 
! Configure port fa0/5 as a trunk link
Switch(config)# interface fa0/5
Switch(config-if)# switchport mode trunk
Switch(config-if)# exit

Step 3: Configure Host IP Allocations

Assign static IP addresses to the connected workstations:

PC1 (VLAN 10):
- IP Address: 192.168.1.10
- Subnet Mask: 255.255.255.0
- Default Gateway: 192.168.1.1
PC2 (VLAN 10):
- IP Address: 192.168.1.20
- Subnet Mask: 255.255.255.0
- Default Gateway: 192.168.1.1
PC3 (VLAN 20):
- IP Address: 192.168.2.10
- Subnet Mask: 255.255.255.0
- Default Gateway: 192.168.2.1
PC4 (VLAN 20):
- IP Address: 192.168.2.20
- Subnet Mask: 255.255.255.0
- Default Gateway: 192.168.2.1

Step 4: Router-on-a-Stick Configuration (R.O.A.S.)

Enable Inter-VLAN Routing by dividing the physical router interface fa0/0 into two distinct logical sub-interfaces. Each sub-interface acts as the default gateway for its assigned VLAN.

Note: The physical interface fa0/0 must not have an IP address assigned to it directly. It must simply be enabled (no shutdown) so the sub-interfaces can process traffic.

text

Router> enable
Router# configure terminal
 
! Bring up the physical interface fa0/0
Router(config)# interface fa0/0
Router(config-if)# no shutdown
Router(config-if)# exit
 
! Configure Sub-Interface for VLAN 10
Router(config)# interface fa0/0.10
Router(config-subif)# encapsulation dot1Q 10
Router(config-subif)# ip address 192.168.1.1 255.255.255.0
Router(config-subif)# exit
 
! Configure Sub-Interface for VLAN 20
Router(config)# interface fa0/0.20
Router(config-subif)# encapsulation dot1Q 20
Router(config-subif)# ip address 192.168.2.1 255.255.255.0
Router(config-subif)# exit

8. Summary

Network devices operate at distinct layers: Repeaters/Hubs (Layer 1) regenerate raw bits; Bridges/Switches (Layer 2) route frames using MAC addresses; Routers (Layer 3) forward packets based on logical IP addresses.
Repeaters regenerate digital signals to overcome line attenuation; they do not amplify analog noise or inspect high-level headers.
Redundancy at Layer 2 introduces packet loops (such as broadcast storms and MAC table instability). The Spanning Tree Protocol (STP) prevents these loops by dynamically disabling redundant paths, leaving a single logical spanning tree active.
VLANs partition physical networks into separate logical broadcast domains, improving network performance, scalability, and security.
Inter-VLAN communication requires a Layer 3 device. The Router-on-a-Stick (R.O.A.S.) architecture uses logical sub-interfaces on a single physical interface to route 802.1Q tagged frames across VLAN boundaries.

Assessed progress

Connecting Devices (Hubs, Bridges, Switches, and Routers)

Connecting Devices (Hubs, Bridges, Switches, and Routers)

1. Five Categories of Connecting Devices

2. Physical Layer Devices: Repeaters and Hubs

Repeaters

Hubs

3. Data Link Layer Devices: Bridges and Two-Layer Switches

Bridges

Learning Bridges

The Loop Problem and the Spanning Tree Protocol (STP)

Spanning Tree Algorithm Steps:

4. Network Layer and Above: Routers, Three-Layer Switches, and Gateways

Routers

Three-Layer Switches

Gateways

5. Backbone Networks

Bus Backbone

Star Backbone

Remote LANs

6. Virtual Local Area Networks (VLANs)

Key Concepts of VLANs:

Port Types in VLAN Configuration

IEEE 802.1Q Frame Tagging

7. Practical Configuration: VLANs and Inter-VLAN Routing

Scenario Topology

Step 1: Switch VLAN Creation

Step 2: Assign Interfaces to VLANs

Step 3: Configure Host IP Allocations

Step 4: Router-on-a-Stick Configuration (R.O.A.S.)

8. Summary

Key Concepts

OSI Layer Device Classification

Learning Bridges & STP

Virtual LANs (VLANs)

Access vs. Trunk Ports

Router-on-a-Stick (R.O.A.S.)

Quick Check

Computer Networks

Computer Networks