Routing Protocols

Routing protocols allow routers to dynamically learn about network paths and share this information with other routers. Understanding these protocols is essential for designing and troubleshooting enterprise networks.

Static vs Dynamic Routing

The first decision in routing design is whether to manually configure routes or let protocols handle it automatically.

Static vs Dynamic Routing

Use Static When:
  • - Small network (few routers)
  • - Single path to destination
  • - Need predictable behavior
Use Dynamic When:
  • - Large or growing network
  • - Multiple paths available
  • - Need automatic failover

Routing Protocol Categories

Routing protocols fall into three main categories based on how they share and calculate route information.

Routing Protocol Categories

Distance Vector

Routers share their entire routing table with neighbors

How it works:

Each router only knows about its neighbors. Information spreads hop-by-hop like a game of telephone.

Think of it like:

Like asking neighbors for directions - they tell you what they know, but not the full picture.

Advantages
  • + Simple implementation
  • + Low memory usage
  • + Easy to configure
Disadvantages
  • - Slow convergence
  • - Routing loops possible
  • - Limited scalability
Protocols:RIPIGRP

Common Routing Protocols

Each protocol has its strengths and ideal use cases. Click on each protocol below to explore its characteristics.

Common Routing Protocols

OSPF

Open Shortest Path First

Type
Link State
Metric
Cost (bandwidth-based)
Updates
On change only
Convergence
Fast
Key Features:
Hierarchical areasFast convergenceNo hop limitOpen standard
Complexity:
SimpleComplex
Best for: Enterprise networks

OSPF Areas

OSPF uses a hierarchical area design to improve scalability. This is one of the key concepts for the CCNA exam.

OSPF Hierarchical Areas

OSPF uses a hierarchical design. Area 0 (backbone) is the center - all other areas must connect to it. Click an area to learn more.

0
Area 0 - Backbone
The backbone area - all other areas must connect here
Area 0
All inter-area traffic flows through the backbone
ABR
Area Border Router connects areas to the backbone
Why Areas?
Reduces routing table size and SPF calculations

Administrative Distance

When a router learns about the same destination from multiple sources, how does it decide which route to use?

Administrative Distance

When a router learns the same route from multiple sources, it uses Administrative Distance to decide which to trust. Lower AD = More trusted.

Directly Connected
0
Static Route
1
EIGRP Summary
5
eBGP
20
EIGRP Internal
90
OSPF
110
IS-IS
115
RIP
120
EIGRP External
170
iBGP
200
Most Trusted (AD: 0)
Least Trusted (AD: 200+)
Quick Memory Tip: Connected=0, Static=1, EIGRP=90, OSPF=110, RIP=120

Convergence

Convergence is when all routers have consistent routing information after a network change.

Fast convergence is critical for:

  • Minimizing downtime during failures
  • Preventing routing loops
  • Maintaining network stability

OSPF and EIGRP converge faster than RIP because they only send updates when changes occur, not periodically.

Routing Tables

A routing table is the result of all the protocol decisions. When forwarding a packet, the router looks up the destination and uses the longest prefix match (most specific route).

Destination      Next Hop        Interface    Metric
10.0.0.0/8       192.168.1.1     eth0         110
172.16.0.0/16    192.168.1.2     eth1         120
192.168.0.0/24   directly        eth2         0
0.0.0.0/0        192.168.1.254   eth0         1

Practice Tasks

Protocol Selection: For a network with 5 routers in a small office, which routing protocol would you recommend? What about for an enterprise with 500 routers across multiple locations?

View Routing Table: On your computer, run netstat -r (Windows) or ip route (Linux) to see your routing table. Can you identify the default gateway?

Research: What is the difference between an Interior Gateway Protocol (IGP) and an Exterior Gateway Protocol (EGP)? Give examples of each.

Summary

  • Static routing is manual and predictable; dynamic routing adapts automatically
  • Distance Vector protocols (RIP) share routing tables with neighbors
  • Link State protocols (OSPF) build complete topology maps
  • Path Vector protocols (BGP) track the full path to prevent loops
  • Administrative Distance determines which source to trust
  • OSPF is the enterprise standard with hierarchical areas
  • BGP connects autonomous systems and runs the internet

Test Your Knowledge

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