Refresher on Routers

• Used to connect different networks together
• Routes traffic between networks using IP Addresses
• Uses intelligent decisions (Routing Protocols) to find the best path for a packet from source to destination
• Break up Broadcast Domains — each interface is its own broadcast domain
• OSI Layer 3 Device
  • Layer 3 = Router
  • Layer 2 = Switch
  • Layer 1 = Hub

Step 1: The Routing Process

• PC1 creates a packet destined for PC2
  • Source IP: 192.168.1.2/24
  • Destination IP: 192.168.0.2/24
• Destination is on a different network → packet sent to PC1’s default gateway
• Default gateway = Router 1’s Ethernet interface (192.168.1.1/24)
• If PC1 doesn’t know Router 1’s MAC address → PC1 sends an ARP request

Step 2: The Routing Process

• Router 1 receives the packet and inspects the destination IP (192.168.0.2)
• Router 1 looks up its routing table to decide where to forward it
• The matching entry: S 192.168.0.0/24 [1/0] via 200.100.100.254
• This is a static route — it sends the packet out the serial interface (200.100.100.1/24) toward Router 2

Step 3: The Routing Process

• Router 2 receives the packet and inspects the destination IP (192.168.0.2)
• It looks up its routing table and finds: C 192.168.0.0/24 is directly connected, FastEthernet0/0
• Directly connected — Router 2 can deliver the packet straight to PC2
• Router 2 uses ARP to find PC2’s MAC and delivers the packet ✅

Static Routing

• The simplest form of routing
• Routes are manually entered by a network administrator
• Ideal for small networks with very few routes that rarely change
  • No routing protocol overhead
• Problematic for larger or changing networks
  • All changes must be made manually
  • Time-consuming and error-prone

Dynamic Routing

• Uses routing protocols to automatically:
  • Populate the router’s routing table
  • Make the most efficient routing decision
  • Update the routing table when the network changes
• Automatic & Hands-Off — all decisions handled by the protocol
• Ideal for larger, more complex networks

Types of Dynamic Routing Protocols

There are three types of dynamic routing protocols:

• Distance-Vector
  • Use hop count as metric (e.g. RIP, EIGRP)
  • Share routing table with direct neighbours
• Link-State
  • Build a complete map of the network (e.g. OSPF)
  • Share link-state advertisements (LSAs) with all routers
• Hybrid
  • Combines features of both (e.g. EIGRP is sometimes classified here)

AS — Autonomous System

• Autonomous = Independent Entity (Organisation)
  • University, Corporation, Government Agency
• Routers are usually part of an Autonomous System (AS)
  • IP routes under common administrative control
• An AS is a connected group of one or more IP prefixes run by one or more network operators with a single, clearly defined routing policy
• Key reference point for understanding IGP and EGP

EGP — Exterior Gateway Protocol

• Used to route between Autonomous Systems
  • Used by Internet Service Providers (ISPs)
• BGP (Border Gateway Protocol)
  • The only EGP in widespread use today
  • Almost all ISPs use BGP as their EGP

Interior Gateway Protocols (IGPs)

There are three types of IGPs:

• Distance-Vector
  • Metric = Hop count (number of routers traversed)
  • Shares routing tables with neighbours every interval
  • Protocols: RIP   EIGRP
• Link-State
  • Builds a complete topology map using LSAs
  • Faster convergence; scales to large networks
  • Protocol: OSPF

Distance-Vector Routing

• Uses distance as the metric for routing decisions
  • Distance = Hop Count
  • Hops = number of routers a packet passes through
• Each router shares its routing table with directly connected neighbours
• Routers only know distances — they don’t have a full map of the network
• Distance-Vector protocols:
  • RIP   Routing Information Protocol
  • EIGRP   Enhanced IGRP

RIP — Routing Information Protocol

• A long-established distance-vector protocol (3 versions)
• Maximum 15 hops to prevent routing loops
  • Does not scale well for large networks
• Sends a full copy of its routing table to directly connected neighbours every 30 seconds
  • Slow convergence — Router 3 may wait up to 90s to learn Router 0’s routes
  • Generates unnecessary traffic and high CPU utilisation

EIGRP — Enhanced IGRP

• A Cisco proprietary protocol — only works on Cisco routers
• Not a pure distance-vector protocol — also uses reliability, bandwidth, load, and delay as metrics
• Often called an advanced distance-vector or hybrid protocol
• Default hop count of 100, maximum of 255
• Supports classless routing and VLSM
• Very fast convergence and highly scalable for large networks

Link-State Routing

• Builds a complete map of the entire network
• Uses Link-State Advertisements (LSAs) to share information:
  • Routers flood LSAs to all other routers
  • Each router builds an identical topology map
• After the initial map is built, routers only communicate when the network changes
• Otherwise, just periodic “hello” packets to confirm neighbours are alive
• Results in faster convergence and better scalability
• Protocol: OSPF   Open Shortest Path First

OSPF — Open Shortest Path First

• Open standard link-state routing protocol
• Well-suited for large networks with multiple redundant paths
• Builds a shortest-path tree (SPF tree) to determine best routes
• Divides larger networks into areas to reduce update traffic
  • Area 0 = Backbone area (all other areas must connect to it)
• Uses “cost” metric based on link speed and state
• Supports classless addressing and VLSM
• Unlimited hop count

Routing Table Components

Every entry in a routing table includes at minimum:

The Default Route

• A static route used when no specific match is found in the routing table
• If there’s no match → forward the packet via the default route
• Commonly called the gateway of last resort
• Represented as:
  • IPv4: 0.0.0.0/0
  • IPv6: ::/0