Routing Protocols and Concepts

Overview

Routing is the process of selecting paths in a network along which to send network traffic. Routers examine the destination IP address of a packet, determine the next-hop, and forward the packet accordingly. This guide covers routing fundamentals, static routing, and dynamic routing protocols.

Prerequisites

Before diving into routing concepts, you should understand:

Network Fundamentals - IP addressing and subnetting
OSI Model - Layer 3 Network Layer operations
Basic TCP/IP concepts and packet forwarding

Routing Fundamentals

What is Routing?

Routing involves two basic activities:

Determining optimal routing paths - Building and maintaining routing tables
Packet forwarding - Moving packets from source to destination through intermediary routers

Routing Table

A routing table contains information about network topology and determines the best path to reach destination networks. Each entry typically includes:

Field	Description	Example
Destination Network	Target network address	192.168.10.0/24
Subnet Mask	Network mask	255.255.255.0
Next Hop	IP address of next router	192.168.1.1
Interface	Outgoing interface	eth0, GigabitEthernet0/1
Metric	Path cost/preference	10, 110
Administrative Distance	Route source trustworthiness	1 (static), 110 (OSPF)

Administrative Distance (AD)

Administrative Distance determines the trustworthiness of a routing protocol. Lower AD values are preferred:

Route Source	AD Value	Use Case
Directly Connected	0	Always preferred
Static Route	1	Manual configuration
EIGRP Summary	5	Cisco proprietary
External BGP (eBGP)	20	Internet routing
Internal EIGRP	90	Cisco proprietary
OSPF	110	Enterprise standard
IS-IS	115	Service provider
RIP	120	Legacy networks
External EIGRP	170	Cisco proprietary
Internal BGP (iBGP)	200	Large enterprises
Unknown/Unreliable	255	Never used

Routing Metrics

Different protocols use different metrics to determine the best path:

Hop Count (RIP) - Number of routers to destination
Bandwidth (OSPF) - Link speed (higher is better)
Delay (EIGRP) - Time to traverse the link
Load (EIGRP) - Link utilization
Reliability (EIGRP) - Error rate
Cost (OSPF) - Calculated from bandwidth: Cost = 100,000,000 / bandwidth (bps)

Static Routing

Static Routing Overview

Static routes are manually configured by network administrators. They provide explicit control over routing decisions and are ideal for small networks or specific routing requirements.

Advantages of Static Routing

Predictable - Routes never change unless manually modified
Secure - No routing protocol advertisements
No overhead - No routing protocol traffic or processing
Simple - Easy to understand and troubleshoot
Precise control - Administrator determines exact paths

Disadvantages of Static Routing

Not scalable - Manual configuration required for each route
No fault tolerance - Routes don't adapt to topology changes
Administrative burden - High maintenance in large networks
Prone to errors - Manual configuration mistakes

Static Route Types

Default Static Route

Points to a gateway for all unknown destinations (0.0.0.0/0):

# Conceptual representation
ip route 0.0.0.0 0.0.0.0 192.168.1.1

Use Case: Small offices with single internet connection

Host Route

Specific route to a single host (/32 for IPv4):

ip route 10.1.1.100 255.255.255.255 192.168.1.1

Use Case: Management access, specific server routing

Network Route

Route to a specific network:

ip route 172.16.0.0 255.255.0.0 192.168.1.1

Use Case: Inter-site connectivity, branch office routing

Floating Static Route

Backup route with higher administrative distance:

ip route 0.0.0.0 0.0.0.0 192.168.1.1       # AD = 1 (primary)
ip route 0.0.0.0 0.0.0.0 192.168.2.1 10    # AD = 10 (backup)

Use Case: Redundant WAN links, failover scenarios

Dynamic Routing Protocols

Dynamic Routing Overview

Dynamic routing protocols automatically discover routes, adapt to topology changes, and select optimal paths. They exchange routing information with neighboring routers to maintain up-to-date routing tables.

Classification

By Scope

Interior Gateway Protocols (IGP) - Within an autonomous system
- RIP, OSPF, EIGRP, IS-IS
Exterior Gateway Protocols (EGP) - Between autonomous systems
- BGP

By Algorithm

Distance Vector - Routes based on distance (hop count) and direction
- RIP, IGRP, EIGRP (hybrid)
Link-State - Each router has complete network topology
- OSPF, IS-IS
Path Vector - Routes based on path attributes and policies
- BGP

Routing Protocol Comparison

Protocol	Type	Metric	Convergence	Scalability	Use Case
RIP	Distance Vector	Hop Count	Slow	Small (max 15 hops)	Legacy, simple networks
EIGRP	Advanced Distance Vector	Composite	Fast	Medium-Large	Cisco-only networks
OSPF	Link-State	Cost (Bandwidth)	Fast	Large	Enterprise standard
IS-IS	Link-State	Cost	Fast	Very Large	Service providers
BGP	Path Vector	Path Attributes	Slow	Internet-scale	ISPs, large enterprises

RIP (Routing Information Protocol)

RIP Overview

RIP is one of the oldest distance-vector routing protocols, using hop count as its metric.

RIP Key Characteristics

Maximum hop count: 15 (16 = unreachable)
Updates: Periodic broadcasts every 30 seconds
Metric: Hop count only
Convergence: Slow (several minutes)
Versions: RIPv1 (classful), RIPv2 (classless, VLSM support)

Advantages

Simple configuration
Low resource requirements
Well-understood and widely supported

Disadvantages

Limited scalability (15 hop maximum)
Inefficient (broadcasts entire table every 30 seconds)
Slow convergence
Hop count doesn't consider bandwidth

When to Use RIP

Small networks (under 15 hops)
Legacy systems requiring compatibility
Test/lab environments
Simple routing requirements

OSPF (Open Shortest Path First)

OSPF Overview

OSPF is a link-state IGP that uses Dijkstra's algorithm to calculate the shortest path. It's the most widely deployed IGP in enterprise networks.

OSPF Key Characteristics

Metric: Cost (based on bandwidth)
Updates: Event-driven (topology changes only)
Hierarchical: Area-based design (Area 0 is backbone)
Scalability: Large networks with proper area design
Convergence: Fast (subsecond to seconds)
Standard: Open standard (RFC 2328)

OSPF Areas

flowchart TB
    subgraph "OSPF Network Design"
        Area0["Area 0<br>(Backbone)<br>Core Routers"]
        Area1["Area 1<br>Regular Area"]
        Area2["Area 2<br>Regular Area"]
        Area3["Area 3<br>Stub Area"]
        
        Area1 --- Area0
        Area2 --- Area0
        Area3 --- Area0
    end
    
    classDef backbone fill:#c83349,stroke:#333,color:#fff
    classDef regular fill:#5b9aa0,stroke:#333,color:#fff
    classDef stub fill:#d6e5fa,stroke:#333
    
    class Area0 backbone
    class Area1,Area2 regular
    class Area3 stub

OSPF Router Types

Internal Router (IR) - All interfaces in same area
Backbone Router (BR) - Interface in Area 0
Area Border Router (ABR) - Connects multiple areas to Area 0
Autonomous System Boundary Router (ASBR) - Connects OSPF to other routing domains

OSPF Area Types

Area Type	LSA Types Allowed	Use Case
Standard Area	All LSAs (1,2,3,4,5)	Default area type
Stub Area	1,2,3 (no Type 5)	Reduce routing table size
Totally Stubby Area	1,2 (no Type 3,4,5)	Cisco proprietary, minimal routes
Not-So-Stubby Area (NSSA)	1,2,3,7	Allow external routes into stub

Advantages of OSPF

Fast convergence
Efficient updates (only changes)
Supports large networks with areas
Vendor-neutral open standard
Load balancing across equal-cost paths
VLSM and CIDR support
Authentication support

Disadvantages of OSPF

Complex configuration
High CPU and memory requirements
Requires careful area design
Challenging to troubleshoot

When to Use OSPF

Enterprise networks (10+ routers)
Requires vendor neutrality
Hierarchical network design
Fast convergence needed
Link state awareness required

EIGRP (Enhanced Interior Gateway Routing Protocol)

EIGRP Overview

EIGRP is Cisco's advanced distance-vector (hybrid) protocol, combining benefits of distance-vector and link-state protocols.

EIGRP Key Characteristics

Metric: Composite (bandwidth, delay, load, reliability)
Updates: Partial, bounded updates (only changes)
Algorithm: DUAL (Diffusing Update Algorithm)
Convergence: Very fast (subsecond)
Scalability: Large networks
Protocol: Cisco proprietary (some open implementations exist)

EIGRP Terminology

Successor - Best route to destination (primary path)
Feasible Successor - Backup route meeting feasibility condition
Feasible Distance (FD) - Best metric to destination
Reported Distance (RD) - Neighbor's metric to destination
Feasibility Condition - RD < FD (guarantees loop-free backup)

Advantages of EIGRP

Extremely fast convergence
Efficient bandwidth usage (bounded updates)
Loop-free topology (DUAL algorithm)
Supports VLSM and CIDR
Flexible metric calculation
Easy configuration
Unequal-cost load balancing

Disadvantages of EIGRP

Cisco proprietary (limited multi-vendor support)
Opaque operation (complex troubleshooting)
Can be memory intensive

When to Use EIGRP

Cisco-only network infrastructure
Requires fast convergence
Large networks with minimal configuration
Unequal-cost load balancing needed

BGP (Border Gateway Protocol)

BGP Overview

BGP is the path-vector protocol that routes traffic across the internet. It connects autonomous systems and makes routing decisions based on policies, paths, and network rules.

BGP Key Characteristics

Metric: Path attributes (AS_PATH, LOCAL_PREF, MED, etc.)
Updates: Incremental, triggered
Scalability: Internet-scale (millions of routes)
Convergence: Slow (minutes)
Standard: Open standard (RFC 4271)
TCP-based: Port 179

BGP Types

eBGP (External BGP) - Between different autonomous systems, AD=20
iBGP (Internal BGP) - Within same autonomous system, AD=200

Path Selection Criteria

BGP uses this order to select best path:

Weight (Cisco proprietary, local to router)
Local Preference (higher is better, within AS)
Locally Originated (routes from this router)
AS_PATH (shorter is better)
Origin Type (IGP > EGP > Incomplete)
MED (Multi-Exit Discriminator) (lower is better, between ASs)
eBGP over iBGP (external preferred)
IGP metric to BGP next-hop
Oldest route (stability)
Lowest Router ID

Advantages of BGP

Extremely scalable (Internet routing)
Policy-based routing control
Loop prevention (AS_PATH)
Supports complex routing policies
Standard protocol

Disadvantages of BGP

Complex configuration and operation
Slow convergence
Requires deep expertise
High memory requirements

When to Use BGP

Multi-homed internet connections
Enterprise connecting to multiple ISPs
Service provider networks
Traffic engineering requirements
Policy-based routing needed

Routing Protocol Selection Guide

Small Networks (1-10 routers)

Recommended: Static routing or OSPF
Why: Simple management, predictable behavior
Alternative: RIP if legacy compatibility required

Medium Networks (10-50 routers)

Recommended: OSPF (multi-vendor) or EIGRP (Cisco-only)
Why: Scalable, fast convergence, efficient
Design: Single OSPF area or EIGRP domain

Large Networks (50+ routers)

Recommended: OSPF with multi-area design
Why: Hierarchical design reduces overhead
Design: Multiple OSPF areas with Area 0 backbone
Alternative: IS-IS for very large networks

Internet Edge

Recommended: BGP
Why: Policy control, multi-homing support
Design: eBGP to ISPs, iBGP internally if needed

Hybrid Scenarios

Many networks use multiple protocols:

Internet --- [BGP] --- Border Router --- [OSPF/EIGRP] --- Internal Network

BGP for external connectivity and policy
OSPF/EIGRP for fast internal routing
Route redistribution at boundary

Troubleshooting Routing

Common Issues

Routes Not Appearing

Check:

Routing protocol enabled on interface
Network statement includes interface subnet
No passive-interface configuration blocking updates
Adjacent router compatibility (protocol version, authentication)

Routing Loops

Symptoms: Traffic circulates endlessly, high CPU, duplicate packets

Causes:

Incorrect static routes
Routing protocol misconfiguration
Redistribution issues

Solutions:

Use split horizon (distance-vector)
Implement route filtering
Check administrative distances

Suboptimal Routing

Symptoms: Traffic takes longer/expensive path