Chapter 6: Routing & Switching

Mastering the protocols and technologies that direct traffic across networks, from routing algorithms to VLAN segmentation.

Introduction

Routing and switching form the backbone of modern network communication. Building on our understanding of IP addressing (Chapter 4) and network protocols (Chapter 5), this chapter explores how data is efficiently directed across networks.

By the end of this chapter, you will understand:

Key routing algorithms (RIP, OSPF, BGP) and their operation
VLANs for network segmentation and MAC address fundamentals
Switching techniques (Store & Forward vs Cut-through)
Static versus dynamic routing approaches
Routing table structure and function
Spanning Tree Protocol for loop prevention
Quality of Service (QoS) mechanisms

Routing Algorithms

RIP (Routing Information Protocol)

RIP is a distance-vector routing protocol that uses hop count as its metric. Key characteristics:

Maximum hop count of 15 (16 = infinity/unreachable)
Updates routing tables every 30 seconds
Uses Bellman-Ford algorithm
Simple to configure but slow convergence

RIP Version 2 Configuration (Cisco IOS)

Router(config)# router rip
Router(config-router)# version 2
Router(config-router)# network 192.168.1.0
Router(config-router)# no auto-summary

Common issues with RIP:

Count-to-infinity problem (solved with split horizon)
Slow convergence in large networks
Lack of support for VLSM in RIPv1

OSPF (Open Shortest Path First)

OSPF is a link-state routing protocol that uses Dijkstra's algorithm to calculate the shortest path. Key features:

Uses cost as metric (based on link bandwidth)
Fast convergence with triggered updates
Supports hierarchical design with areas
Requires more CPU/memory than RIP but scales better

OSPF Areas

Backbone Area (Area 0): Connects all other areas
Regular Areas: Connect to backbone via ABRs
Stub Areas: Don't receive external routes

OSPF Configuration (Cisco IOS)

Router(config)# router ospf 1
Router(config-router)# network 10.0.0.0 0.255.255.255 area 0

BGP (Border Gateway Protocol)

BGP is the path-vector protocol that routes traffic between autonomous systems (AS) on the internet. Key aspects:

Makes routing decisions based on paths, network policies, and rules
Uses TCP port 179 for reliable communication
Very scalable but complex to configure
Primary protocol for internet backbone routing

BGP Peering Configuration

Router(config)# router bgp 65001
Router(config-router)# neighbor 203.0.113.1 remote-as 65002
Router(config-router)# network 192.0.2.0

VLANs and MAC Addresses

Virtual LANs (VLANs)

VLANs logically segment networks at Layer 2, providing:

Improved security through isolation
Better performance by limiting broadcast domains
Simplified network management

VLAN tagging (IEEE 802.1Q) adds a 4-byte tag to Ethernet frames:

12-bit VLAN ID (supports 4094 VLANs)
3-bit priority field for QoS
Native VLAN (untagged traffic)

VLAN Configuration (Cisco IOS)

Switch(config)# vlan 10
Switch(config-vlan)# name Sales
Switch(config)# interface fastEthernet 0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10

Common VLAN Issues

VLAN mismatch on trunk links
Native VLAN configuration errors
Missing VLANs on some switches

Troubleshoot with: show vlan brief, show interfaces trunk

MAC Addresses

MAC (Media Access Control) addresses are 48-bit hardware addresses assigned to network interfaces. Key points:

First 24 bits: OUI (Organizationally Unique Identifier)
Last 24 bits: Device-specific identifier
Represented in hexadecimal (e.g., 00:1A:2B:3C:4D:5E)
Used for Layer 2 frame delivery

MAC Address Table

Switches maintain a MAC address table that maps MAC addresses to switch ports:

Switch# show mac address-table
          Mac Address Table
-------------------------------------------
Vlan    Mac Address       Type        Ports
----    -----------       ----        -----
 10    001a.2b3c.4d5e    DYNAMIC     Fa0/1

MAC Address Learning

Switch receives frame on a port
Notes source MAC address and port
Adds entry to MAC address table
Forwards frame based on destination MAC

Switching Techniques

Store & Forward

The switch:

Receives the entire frame before forwarding
Performs CRC error checking
Discards frames with errors
Higher latency but more reliable

Best for: Networks where reliability is critical

Cut-through

The switch:

Begins forwarding after reading destination MAC
Doesn't store entire frame
No error checking
Lower latency but may forward bad frames

Best for: High-performance, low-latency networks

Comparison Table

Feature	Store & Forward	Cut-through
Latency	Higher	Lower
Error Checking	Yes	No
Memory Usage	More	Less
Frame Size	Stores entire frame	Only reads header

Static vs Dynamic Routing

Static Routing

Manually configured routes:

Simple to implement
No protocol overhead
Requires manual updates
Doesn't adapt to topology changes

Configuration Example

Router(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2

Dynamic Routing

Routes learned via routing protocols:

Adapts to network changes
More complex to configure
Protocol overhead
Automatic updates

When to Use

Large networks, changing topologies, redundant paths

Choosing Between Static and Dynamic

Use static routing for small networks with simple topologies
Use dynamic routing for large networks with multiple paths
Hybrid approach: Static for edge routes, dynamic for core

Routing Tables

Routing tables contain the information routers use to forward packets. Key fields:

Destination Network: IP address and subnet mask
Next Hop: IP address of next router
Interface: Outgoing interface
Metric: Cost to reach destination
Route Source: How route was learned (static, RIP, OSPF, etc.)

Sample Routing Table (Cisco IOS)

Router# show ip route
Codes: C - connected, S - static, R - RIP, O - OSPF

O    192.168.2.0/24 [110/20] via 10.0.0.2, FastEthernet0/1
C    192.168.1.0/24 is directly connected, FastEthernet0/0
S    203.0.113.0/24 [1/0] via 198.51.100.1

Route Selection Process

Longest prefix match (most specific route)
Lowest administrative distance
Lowest metric

Administrative Distance

Directly connected: 0
Static route: 1
OSPF: 110
RIP: 120

Spanning Tree Protocol (STP)

STP (IEEE 802.1D) prevents loops in switched networks by:

Electing a root bridge
Calculating shortest paths to root
Blocking redundant paths

STP Port States

Blocking: No forwarding, listens to BPDUs
Listening: Prepares to forward but doesn't learn MACs
Learning: Learns MAC addresses but doesn't forward
Forwarding: Normal operation

Root Bridge Election

Compare Bridge IDs (Priority + MAC address)
Lower Bridge ID wins
Default priority: 32768

Force a root bridge:

Switch(config)# spanning-tree vlan 1 root primary

STP Topology Visualization

Diagram showing root bridge, designated ports, and blocked ports in a switched network.

STP Variants

RSTP (802.1w): Rapid Spanning Tree - faster convergence
MSTP (802.1s): Multiple Spanning Tree - maps VLANs to instances
PVST+: Per-VLAN Spanning Tree (Cisco proprietary)

Quality of Service (QoS)

QoS mechanisms prioritize network traffic to ensure performance for critical applications:

Classification

Identify traffic types (e.g., VoIP, video, data) using:

DSCP (Differentiated Services Code Point)
802.1p priority bits
ACLs

Queuing

Manage how packets are buffered:

Priority Queuing (PQ)
Weighted Fair Queuing (WFQ)
Class-Based WFQ (CBWFQ)

Policing/Shaping

Control bandwidth usage:

Policing: Drops excess traffic
Shaping: Buffers excess traffic

QoS Configuration Example (VoIP Priority)

Switch(config)# class-map match-any VOICE
Switch(config-cmap)# match dscp ef
Switch(config-cmap)# match ip rtp 16384 16383
Switch(config)# policy-map QOS-POLICY
Switch(config-pmap)# class VOICE
Switch(config-pmap-c)# priority percent 30
Switch(config)# interface FastEthernet0/1
Switch(config-if)# service-policy output QOS-POLICY

Practical Examples

Example 1: Configuring OSPF in Packet Tracer

Step-by-step guide to setting up a multi-area OSPF network:

Steps:

Add three routers and connect them (serial or Ethernet links)
Configure IP addresses on all interfaces

Enable OSPF on each router:

Router(config)# router ospf 1
Router(config-router)# network 10.0.0.0 0.255.255.255 area 0
Router(config-router)# network 192.168.1.0 0.0.0.255 area 1

Verify OSPF neighbors:
```
Router# show ip ospf neighbor
```
Check routing tables to confirm OSPF routes

Troubleshooting Tips

Verify interfaces are up/up with show ip interface brief
Check OSPF is enabled on correct networks
Ensure area configurations match on connected routers
Use debug ip ospf events for detailed troubleshooting

Example 2: Visualizing OSPF Network with HTML Canvas

Interactive diagram showing an OSPF network with areas:

Interactive diagram showing routers in different OSPF areas with link costs.

OSPF Network Components

Backbone Router: In Area 0, connects to other areas
Area Border Router (ABR): Connects Area 0 to other areas
Internal Router: All interfaces in same area
Designated Router (DR): Elected on multi-access networks

Summary

In this chapter, we've explored the core concepts of routing and switching:

Routing Protocols: RIP (distance-vector), OSPF (link-state), BGP (path-vector)
VLANs: Logical segmentation at Layer 2 using 802.1Q tagging
Switching: Store & Forward (reliable) vs Cut-through (low latency)
Routing Methods: Static (manual) vs Dynamic (protocol-based)
Routing Tables: Contain destination networks, next hops, and metrics
STP: Prevents loops by blocking redundant paths
QoS: Prioritizes critical traffic through classification and queuing

Key Takeaways

Choose routing protocols based on network size and requirements
Use VLANs to improve security and performance
Understand how switches make forwarding decisions
Implement STP to prevent switching loops
Configure QoS for voice and video traffic

Routing & Switching | Computer Networks: From Scratch to Mastery