Backbone Networks and Topologies

Modern computer networks rely on hierarchical interconnection strategies to bridge individual Local Area Networks (LANs) into unified, scalable enterprise networks. This module covers the theoretical foundations of network interconnection, spanning tree loop resolution, backbone topologies, and Virtual LAN (VLAN) design.

1. Network Interconnection Devices

Interconnection components operate across different layers of the Open Systems Interconnection (OSI) reference model. These devices range from simple Physical-layer signal regenerators to application-aware gateways.

plaintext

+--------------------------------------------------------+
|                 OSI Layer Devices                      |
+-------------------+------------------------------------+
| Application       | Gateway                            |
| Transport/Network | Router / Three-Layer Switch        |
| Data Link         | Bridge / Two-Layer Switch          |
| Physical          | Repeater / Active Hub / Passive Hub|
+-------------------+------------------------------------+

1.1 Physical Layer Devices

At the lowest layer of the network architecture, devices forward electrical, optical, or electromagnetic signals without evaluating frame structures or logical addresses.

Passive Hubs: Serve purely as physical connection points. They do not regenerate or clean signals; they simply distribute incoming electrical signals to all connected physical lines.
Active Hubs / Repeaters: A repeater connects distinct physical segments of a single LAN. It is critical to recognize that a repeater is a regenerator, not an amplifier.
- An amplifier boosts both the desired signal and the accumulated channel noise, which can result in a degraded signal-to-noise ratio (SNR).
- A repeater receives the corrupted, attenuated digital signal, parses the high and low voltage states, reconstructs the original bit pattern, and transmits a clean, full-strength signal.
- A repeater operates blindly; it has no filtering capabilities, does not inspect MAC addresses, and forwards every single bit.

1.2 Data Link Layer Devices

Data Link layer devices parse frame headers and make forwarding decisions based on physical hardware addresses (MAC addresses).

Bridges: A bridge connects separate physical LAN segments, creating a larger logical network. Unlike repeaters, bridges contain internal logic and maintain a forwarding table to filter, forward, or flood frames.
Learning Bridges: Rather than requiring manual table configuration, a learning bridge dynamically populates its database by monitoring incoming traffic. It records the source MAC address of an incoming frame and maps it to the receiving port.

text

                  Learning Bridge Function
                +--------------------------+
                | MAC Address | Port Link  |
                +-------------+------------+
                |  A1-B2-C3   |   Port 1   |
                |  D4-E5-F6   |   Port 2   |
                +-------------+------------+
                             |
    [Host A]                 v                 [Host B]
  (A1-B2-C3) ------------ [Port 1] ----------- (D4-E5-F6)
                           [Port 2]

When a frame arrives:

The bridge inspects the Source MAC Address. If the entry does not exist, it adds it to the forwarding table along with the ingress port and a timestamp.
The bridge inspects the Destination MAC Address:
- Case A: Found in Table. If the destination port matches the ingress port, the frame is filtered (discarded) because the destination resides on the same segment. If it matches a different port, the frame is forwarded to that port.
- Case B: Not Found in Table (Unknown Unicast). The bridge floods the frame out of all ports except the port on which it was received.
- Case C: Broadcast/Multicast. The frame is flooded out of all ports except the source port.
Two-Layer Switches: A multi-port, high-speed learning bridge. Switches utilize dedicated hardware (Application-Specific Integrated Circuits or ASICs) to execute frame forwarding in silicon at wire speed, offering dedicated collision domains per port.

1.3 Network and Higher-Layer Devices

These devices operate beyond the frame layer, managing logical addressing and protocol translation.

Routers: Layer 3 (Network Layer) routing devices that connect entirely independent networks (e.g., joining an enterprise Ethernet LAN to a Wide Area Network). Routers read logical IP addresses and execute routing algorithms (e.g., OSPF, BGP) to route packets across subnets. Crucially, routers block Layer 2 broadcasts.
Three-Layer Switches: High-performance LAN switches capable of executing Layer 3 routing at wire-speed using ASIC hardware.
Gateways: Protocol converters operating up to the Application Layer. They translate formatting, syntax, and protocol differences between completely incompatible architectures (e.g., translating between an IP network and an legacy SNA mainframe architecture).

2. Spanning Tree Protocol (STP) in Bridged LANs

To ensure high availability, enterprise networks introduce physical redundancy by deploying parallel paths between switches and bridges. However, introducing physical loops into a Layer 2 network without loop-mitigation mechanisms leads to catastrophic failures.

2.1 The Loop Problem

Consider two bridges connecting two separate LAN segments:

text

               +---------------+
               |   Bridge B1   |
         +-----+               +-----+
         |     +---------------+     |
   [ LAN 1 ]                       [ LAN 2 ]
         |     +---------------+     |
         +-----+   Bridge B2   +-----+
               |               |
               +---------------+

If Host $A$ on LAN 1 transmits a broadcast frame:

Both Bridge $B_1$ and Bridge $B_2$ receive the broadcast.
Since it is a broadcast, both bridges copy and forward the frame onto LAN 2.
Bridge $B_1$ detects the broadcast on LAN 2 (originally sent by ) and forwards it back to LAN 1.

2.2 Graph Representation and the Spanning Tree Algorithm

To resolve this, networks run the Spanning Tree Protocol (STP, IEEE 802.1D). The network of bridges and LANs is modeled as an undirected graph $G = (V, E)$ , where:

$V = \{B_1, B_2, \dots, B_n\} \cup \{L_1, L_2, \dots, L_m\}$ is the set of vertices (representing bridges and LAN segments).

The objective of STP is to construct a minimum spanning tree $T \subset G$ such that:

$T$ contains all vertices (meaning all LANs remain reachable).
There are no cycles (loops) in $T$ .
The path cost from any node to a designated Root Bridge is minimized.

text

     Physical Loop Graph (G)             Logical Spanning Tree (T)
         [Root Bridge B1]                    [Root Bridge B1]
          /            \                      /            \
     [LAN 1]          [LAN 2]            [LAN 1]          [LAN 2]
          \            /                      \          
         [Backup Bridge B2]                  [Backup Bridge B2]
                                              (Link to LAN 2 blocked)

The algorithm operates through three distinct phases:

1. Root Bridge Election

Bridges exchange specialized configuration messages called Bridge Protocol Data Units (BPDUs). Each bridge has a unique Bridge ID (BID), which consists of a configurable Priority value and the bridge's physical MAC address:

$\text{BID} = \text{Priority (2 bytes)} \ || \ \text{MAC Address (6 bytes)}$

The bridge with the lowest BID is elected as the Root Bridge of the network.

2. Root Port Selection

Each non-root bridge determines its Root Port (RP). The Root Port is the individual port on the bridge that offers the lowest path cost back to the Root Bridge. Path cost is inversely proportional to link bandwidth:

$\text{Link Cost} \propto \frac{1}{\text{Bandwidth}}$

For instance, a standard $1 \text{ Gbps}$ link has a lower path cost than a $100 \text{ Mbps}$ link.

3. Designated Port Selection

For each LAN segment, the connected bridges negotiate to elect a single Designated Port (DP). The Designated Port is the port on the bridge that provides the lowest path cost from that LAN segment back to the Root Bridge. All other ports attached to that segment that are neither Root Ports nor Designated Ports are placed into the Blocking State.

Through this process, redundant physical paths are logically disabled (blocked), eliminating loops while remaining available to take over if a primary link fails.

3. Backbone Network Topologies

A backbone network provides a high-capacity link to interconnect separate Local Area Networks. A defining characteristic of backbone networks is that no individual client station is directly connected to the backbone physical media; the backbone connects network nodes (such as switches or routers) rather than end host systems.

3.1 Bus Backbone

In a bus backbone topology, the backbone itself is configured as a physical bus structure, typically using high-performance coaxial cabling or fiber trunks.

text

   +--------+      +--------+      +--------+
   | Switch |      | Switch |      | Switch |
   +---+----+      +---+----+      +---+----+
       |               |               |
=======+===============+===============+=======  <-- High-Speed Bus Backbone

Characteristics: Centralized linear architecture.
Operations: Bridges or switches connect their local star-wired LANs directly to this central bus using transceivers.
Limitations: A single break along the central bus backbone segment splits the network into two isolated partitions.

3.2 Star Backbone

In a star backbone topology, the backbone consists of a single central switch that acts as the hub.

text

               +----------------------+
               | Central Switch (Hub) |  <-- Star Backbone
               +--+----------------+--+
                  |                |
         +--------+                +--------+
         |                                  |
    +----+---+                         +----+---+
    | Switch | [LAN 1]                 | Switch | [LAN 2]
    +--------+                         +--------+

Characteristics: The backbone is encapsulated within a high-performance Layer 2 or Layer 3 central switch.
Operations: Each local LAN switch connects to the central switch via a point-to-point link.
Advantages: Highly modular, easy to troubleshoot, and provides superior throughput, as traffic is switched internally through the central backplane rather than traversing a shared bus.

3.3 Connecting Remote LANs

When LANs are separated by large geographic distances, we use remote bridges connected via point-to-point WAN links.

text

 +-------+     +------------+      WAN Link      +------------+     +-------+
 | LAN A +-----+ Bridge (L) +====================+ Bridge (R) +-----+ LAN B |
 +-------+     +------------+  (Point-to-Point)  +------------+     +-------+

In this architecture, the point-to-point link (such as a leased fiber line or digital microwave link) acts as a virtual LAN segment connecting the two remote bridges, allowing them to forward frames across geographic boundaries.

4. Virtual Local Area Networks (VLANs)

Traditional LANs are defined by physical cabling; all hosts connected to the same physical switch or cascaded hub network belong to the same broadcast domain. A Virtual Local Area Network (VLAN) allows a network administrator to segment a physical switch network into separate logical networks using software.

text

    Physical Infrastructure                   Logical VLANs
     +------------------+                 +--------------------+
     |      Switch      |                 | VLAN 10 (SALES)    |
     +--+--+--+--+--+--+                  |  - PC1             |
        |  |  |  |  |  |                  |  - PC2             |
       PC1 PC2 PC3 PC4 PC5                +--------------------+
      [V10] [V20] [V10] [V20]             | VLAN 20 (IT)       |
                                          |  - PC3             |
                                          |  - PC4             |
                                          +--------------------+

4.1 Key Characteristics and Benefits

Broadcast Domain Control: VLANs restrict broadcast traffic. A broadcast packet generated by a host in VLAN 10 is received only by other members of VLAN 10. This significantly reduces unnecessary broadcast traffic across the network.
Security: By isolating host groups at Layer 2, sensitive departments (e.g., Finance) cannot sniff or access packets from other departments (e.g., Guest Access) without passing through a router or firewall.
Logical Grouping: Port allocation is configured in software. If an employee moves to a different physical office, their computer can remain in the same logical VLAN without rewiring.

4.2 Switch Port Membership and Frame Tagging

Managed switches categorize their physical ports into two primary types:

Access Ports: Assigned to a single, specific VLAN. Standard end devices (like PCs and printers) connect to access ports. These devices send and receive standard, untagged Ethernet frames.
Trunk Ports: Configured to carry traffic for multiple VLANs across a single physical link (such as a link between two switches or a switch and a router).
- To keep track of which frame belongs to which VLAN, switches use the IEEE 802.1Q standard to insert a 4-byte VLAN tag into the Ethernet frame header at egress. This tag is then removed when forwarding the frame out of an access port to an end device.

plaintext

       Standard Ethernet Frame Header:
       +---------+---------+------+----------+
       | Dest MAC| Src MAC | Type | Payload  |
       +---------+---------+------+----------+
 
       802.1Q Tagged Ethernet Frame Header:
       +---------+---------+===========+------+----------+
       | Dest MAC| Src MAC | 802.1Q Tag| Type | Payload  |
       +---------+---------+===========+------+----------+
                            \         /
                             VLAN ID (12 bits)

The 12-bit VLAN ID field allows for a maximum of:

$2^{12} - 2 = 4094 \text{ usable VLANs}$

5. Hands-On Configuration: VLANs and Inter-VLAN Routing

Because each VLAN is its own logical subnet and broadcast domain, hosts in different VLANs cannot communicate directly at Layer 2. To allow communication between different VLANs, a Layer 3 routing device must route traffic between them.

The following guide details how to configure two VLANs on a Cisco switch and set up Router-on-a-Stick (ROAS) on a Cisco router using a single physical interface.

5.1 Network Topology and Addressing Table

text

       [PC 1] (192.168.1.10) -- Port fa0/1 \
                                            +-- [Switch] (fa0/5) === Trunk === (fa0/0) [Router]
       [PC 3] (192.168.2.10) -- Port fa0/3 /

| Device | Interface / Port | VLAN Assignment | IP Address | Subnet Mask | Default Gateway | | :--- | :--- | :--- | :--- | :--- | :--- | | PC1 | Fa0/1 | VLAN 10 (SALES) | 192.168.1.10 | 255.255.255.0 | 192.168.1.1 | | PC2 | Fa0/2 | VLAN 10 (SALES) | 192.168.1.20 | 255.255.255.0 | 192.168.1.1 | | PC3 | Fa0/3 | VLAN 20 (IT) | 192.168.2.10 | 255.255.255.0 | 192.168.2.1 | | PC4 | Fa0/4 | VLAN 20 (IT) | 192.168.2.20 | 255.255.255.0 | 192.168.2.1 |

5.2 Step-by-Step Configuration Commands

Step 1: Initialize and Name VLANs on the Switch

Enter global configuration mode to instantiate the logical VLAN structures and assign names.

cisco

Switch> enable
Switch# configure terminal
 
! Instantiate VLAN 10 and name it SALES
Switch(config)# vlan 10
Switch(config-vlan)# name SALES
Switch(config-vlan)# exit
 
! Instantiate VLAN 20 and name it IT
Switch(config)# vlan 20
Switch(config-vlan)# name IT
Switch(config-vlan)# exit

Step 2: Assign Access Ports to Respective VLANs

Configure the access ports connected to the client machines. This step maps physical switch ports to logical VLAN domains.

cisco

! Configure Ports fa0/1 and fa0/2 for VLAN 10
Switch(config)# interface range fa0/1 - 2
Switch(config-if-range)# switchport mode access
Switch(config-if-range)# switchport access vlan 10
Switch(config-if-range)# exit
 
! Configure Ports fa0/3 and fa0/4 for VLAN 20
Switch(config)# interface range fa0/3 - 4
Switch(config-if-range)# switchport mode access
Switch(config-if-range)# switchport access vlan 20
Switch(config-if-range)# exit

Step 3: Configure the Switch Trunk Port

Port fa0/5 connects the switch directly to the router's physical interface. It must be configured as a trunk link to carry tagged traffic for both VLAN 10 and VLAN 20.

cisco

Switch(config)# interface fa0/5
Switch(config-if)# switchport mode trunk
Switch(config-if)# exit

Step 4: Configure Router-on-a-Stick (ROAS) Inter-VLAN Routing

On the router, we enable the physical link without assigning it an IP address. We then partition the link into logical sub-interfaces, assigning each sub-interface a VLAN tag encapsulation type and its corresponding IP gateway address.

cisco

Router> enable
Router# configure terminal
 
! Bring up the physical interface without an IP address
Router(config)# interface fa0/0
Router(config-if)# no shutdown
Router(config-if)# exit
 
! Create logical sub-interface for VLAN 10
Router(config)# interface fa0/0.10
Router(config-subif)# description Default Gateway for SALES
Router(config-subif)# encapsulation dot1Q 10
Router(config-subif)# ip address 192.168.1.1 255.255.255.0
Router(config-subif)# exit
 
! Create logical sub-interface for VLAN 20
Router(config)# interface fa0/0.20
Router(config-subif)# description Default Gateway for IT
Router(config-subif)# encapsulation dot1Q 20
Router(config-subif)# ip address 192.168.2.1 255.255.255.0
Router(config-subif)# end

5.3 Verification and Testing

After completing these configurations, you can verify your connectivity using the following diagnostic steps:

Intra-VLAN Verification: Run a ping command from PC1 (192.168.1.10) to PC2 (192.168.1.20). The ping should succeed immediately, as both devices belong to the same broadcast domain (VLAN 10) and do not require routing.
Inter-VLAN Verification: Run a ping command from PC1 (192.168.1.10) to PC3 (192.168.2.10).
- The packet is first sent to PC1's default gateway (192.168.1.1) at the router's sub-interface fa0/0.10.
- The router receives the packet, strips the VLAN 10 tag, looks up the destination network in its routing table, and routes the packet to sub-interface fa0/0.20.
- The router encapsulates the packet in an 802.1Q frame tagged with VLAN ID 20 and sends it down the trunk link to the switch.
- The switch receives the tagged frame, identifies it as belonging to VLAN 20, strips the tag, and forwards it to PC3 via the access port fa0/3.
- The ping should succeed, confirming that inter-VLAN routing is working correctly.

Summary

Repeaters and Hubs operate at the Physical Layer (Layer 1). A repeater is a signal regenerator that cleans up digital signal degradation, whereas an amplifier simply boosts both the signal and any accumulated noise.
Bridges and Switches operate at the Data Link Layer (Layer 2). They use a dynamic lookup table (MAC-to-Port) to filter and forward frames, isolating collision domains.
Spanning Tree Protocol (STP) prevents broadcast storms and bridge table corruption in networks with redundant physical loops by logically disabling redundant links.
Backbone Networks connect multiple independent LANs. In a backbone network, no end hosts are directly connected to the backbone media itself. Common topologies include Bus, Star, and Remote Backbones.
VLANs partition physical switches into logical software-defined broadcast domains. An access port connects directly to an end host and carries untagged traffic, while a trunk port carries tagged traffic for multiple VLANs across switches using the IEEE 802.1Q protocol.
Inter-VLAN routing is required to route traffic between different VLAN subnets, often implemented using the space-efficient Router-on-a-Stick (ROAS) architecture.

Assessed progress

Backbone Networks and Topologies

Backbone Networks and Topologies

1. Network Interconnection Devices

1.1 Physical Layer Devices

1.2 Data Link Layer Devices

1.3 Network and Higher-Layer Devices

2. Spanning Tree Protocol (STP) in Bridged LANs

2.1 The Loop Problem

2.2 Graph Representation and the Spanning Tree Algorithm

1. Root Bridge Election

2. Root Port Selection

3. Designated Port Selection

3. Backbone Network Topologies

3.1 Bus Backbone

3.2 Star Backbone

3.3 Connecting Remote LANs

4. Virtual Local Area Networks (VLANs)

4.1 Key Characteristics and Benefits

4.2 Switch Port Membership and Frame Tagging

5. Hands-On Configuration: VLANs and Inter-VLAN Routing

5.1 Network Topology and Addressing Table

5.2 Step-by-Step Configuration Commands

Step 1: Initialize and Name VLANs on the Switch

Step 2: Assign Access Ports to Respective VLANs

Step 3: Configure the Switch Trunk Port

Step 4: Configure Router-on-a-Stick (ROAS) Inter-VLAN Routing

5.3 Verification and Testing

Summary

Key Concepts

Regeneration vs. Amplification

Learning Bridges

Spanning Tree Protocol (STP)

Backbone Networks

Virtual Local Area Networks (VLANs)

Quick Check

Computer Networks

Computer Networks