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IP Routing and Routing Protocols

Definition

IP Routing is the process of forwarding IP packets from one network to another using routers. Routers determine the best path to the destination network by consulting their routing tables.

Key Points

What is IP Routing?

• IP routing allows communication between different networks.
• Routers examine the destination IP address of each packet.
• Routers determine the best next-hop using the routing table.
• The packet is then forwarded toward its destination.

Example

PC A
192.168.1.10
     │
     │
Router R1
     │
     │
Router R2
     │
PC B
10.1.1.20

The routers forward the packet until it reaches the destination network.

Routing Table

Definition

A Routing Table is a database maintained by a router that contains information about reachable networks and the paths used to reach them.

Characteristics

• Stored in RAM.
• Used to determine the next hop.
• Updated manually (static routing) or automatically (dynamic routing).

Information Stored in a Routing Table

Typical entries include:

• Destination network
• Subnet mask (prefix)
• Next-hop IP address
• Exit interface
• Routing protocol
• Administrative Distance (AD)
• Metric

Types of Routing

There are three common types of routing:

1. Static Routing
2. Default Routing
3. Dynamic Routing

Static Routing

Definition

Static Routing is a routing method where the network administrator manually configures routes on every router.

The router does not learn routes automatically.

Advantages

• Low CPU usage
• No routing protocol traffic (no bandwidth overhead)
• Better security
• Simple for very small networks
• Predictable routing behavior

Disadvantages

• Manual configuration required
• Difficult to maintain
• Poor scalability
• Every topology change requires manual updates
• Not suitable for large networks

Static Route Configuration

Using Next-Hop IP

Router(config)# ip route <destination-network> <subnet-mask> <next-hop-IP>

Example

R1(config)# ip route 192.168.10.0 255.255.255.0 192.168.20.1

Explanation

• ip route → Creates a static route.
• 192.168.10.0 → Destination network.
• 255.255.255.0 → Subnet mask.
• 192.168.20.1 → Next-hop router.

Using Exit Interface

Router(config)# ip route <destination-network> <subnet-mask> <exit-interface>

Example

R1(config)# ip route 192.168.10.0 255.255.255.0 Serial2/0

The router forwards packets through the specified interface.

Verify Static Routes

show ip interface brief

Displays interface status.

show ip route

Displays the routing table.

show ip route static

Displays only static routes.

show running-config | include ip route

Shows configured static routes.

show ip protocols

Displays routing protocol information.

show ip route <network> <mask>

Displays a specific route.

Note: In networking, a Network ID (NID) is often referred to as a prefix.

Default Routing

Definition

A Default Route is used when the router does not know a specific route to the destination network.

It is commonly configured on stub routers.

Stub Router

A Stub Router has only one path leading out of the local network.

Default Route Configuration

Router(config)# ip route 0.0.0.0 0.0.0.0 <next-hop-IP | exit-interface>

Example

R1(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1

This route matches any unknown destination.

Verify Default Routing

debug ip packets

Displays packet forwarding information.

debug ip routing

Displays routing updates and routing decisions.

Dynamic Routing

Definition

Dynamic Routing uses routing protocols to automatically discover networks and exchange routing information between routers.

Routers update their routing tables whenever the network topology changes.

Functions of Dynamic Routing Protocols

• Discover neighboring routers
• Exchange routing information
• Build routing tables
• Update routes automatically
• Determine the best path

Advantages

• Automatic updates
• Suitable for large networks
• Easier administration
• Supports redundant paths
• Automatically adapts to failures

Disadvantages

• Uses CPU resources
• Consumes network bandwidth
• More complex than static routing

Routed Protocol vs Routing Protocol

Routed Protocol

Definition

A Routed Protocol carries user data across networks.

Examples:

• IPv4
• IPv6
• IPX
• AppleTalk

Routing Protocol

Definition

A Routing Protocol allows routers to exchange routing information and determine the best paths.

Examples:

• RIP
• OSPF
• EIGRP
• IS-IS
• BGP

Comparison

Components of a Routing Protocol

1. Algorithm

Determines the best route to a destination.

Examples:

• Hop Count
• Cost
• Bandwidth
• Delay

2. Routing Messages

Used to:

• Discover neighbors
• Exchange routing information
• Update routing tables

Classification of Routing Protocols

Routing protocols are divided into:

Routing Protocols
│
├── Interior Gateway Protocols (IGP)
│     ├── Distance Vector
│     ├── Link State
│     └── Hybrid
│
└── Exterior Gateway Protocols (EGP)
      └── BGP

Interior Gateway Protocol (IGP)

Definition

IGP protocols operate within a single organization or autonomous system (AS).

Examples:

• RIP
• OSPF
• EIGRP
• IS-IS

Exterior Gateway Protocol (EGP)

Definition

EGP protocols exchange routing information between different autonomous systems, such as between Internet Service Providers (ISPs).

Example:

• BGP (Border Gateway Protocol)

Distance Vector Routing Protocols

Definition

Distance Vector protocols determine the best path primarily based on distance, commonly measured by hop count.

Examples:

• RIP
• IGRP (Cisco proprietary; obsolete)

Characteristics

• Uses Hop Count
• Periodically sends the entire routing table
• Sometimes called “Routing by Rumor”
• Simple but slower to converge

Hop Count

Every time a packet passes through a router, it counts as one hop.

Example:

PC
 │
R1
 │
R2
 │
R3
 │
Server

Hop Count = 3

Link-State Routing Protocols

Definition

Link-State protocols maintain a complete view of the network topology and calculate the shortest path using the Shortest Path First (SPF) algorithm.

Examples:

• OSPF
• IS-IS

Router Databases

A Link-State router maintains:

1. Neighbor Table
2. Link-State Database (network topology)
3. Routing Table

Characteristics

• Sends updates only when changes occur.
• Builds a complete network map.
• Uses Cost as its metric (OSPF).
• Fast convergence.

Hybrid Routing Protocols

Definition

Hybrid protocols combine features of both Distance Vector and Link-State routing.

Example:

• EIGRP (Enhanced Interior Gateway Routing Protocol)

Characteristics

• Fast convergence
• Efficient updates
• Supports multiple metrics
• Cisco proprietary in its original implementation

Distance Vector vs Link-State

Classful Routing Protocols

Definition

Classful routing protocols do not include subnet mask information in routing updates.

Requirements:

• All subnets must use the same subnet mask.

Examples:

• RIPv1
• IGRP

Classless Routing Protocols

Definition

Classless routing protocols include subnet mask information in routing updates.

Advantages:

• Support Variable Length Subnet Masks (VLSM)
• Support CIDR (Classless Inter-Domain Routing)

Examples:

• RIPv2
• OSPF
• EIGRP
• IS-IS

Classful vs Classless

Metric

Definition

A Metric is a value used by a routing protocol to determine the best path to a destination.

The route with the lowest metric is generally preferred.

Metrics Used by Routing Protocols

Administrative Distance (AD)

Definition

Administrative Distance (AD) measures the trustworthiness of the source of routing information.

• Range: 0–255
• Lower AD = More trusted
• AD 255 = Route is never used

Default Administrative Distance Values

Example

If a router learns the same route through both OSPF and RIP:

The router selects the OSPF route because it has the lower Administrative Distance.

Gateway of Last Resort

Definition

The Gateway of Last Resort is another name for the default route.

When no matching route exists in the routing table, packets are forwarded to the default gateway.

Example:

Known Route?
     │
 ┌───┴────┐
 │        │
Yes      No
 │        │
Forward   Use Default Route

Example / Code

Configure a Static Route

R1(config)# ip route 10.10.10.0 255.255.255.0 192.168.1.2

Explanation

• Creates a route to the 10.10.10.0/24 network.
• Packets are forwarded to the next-hop router at 192.168.1.2.

Configure a Default Route

R1(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1

Explanation

• Creates a Gateway of Last Resort.
• Used whenever no specific route matches the destination.

Display the Routing Table

R1# show ip route

Explanation

Displays:

• Connected routes
• Static routes
• Dynamic routes
• Default route
• Route metrics and administrative distances

Common Mistakes

• Confusing routed protocols (e.g., IPv4) with routing protocols (e.g., OSPF).

• Assuming static routes update automatically.

• Forgetting to update static routes after topology changes.

• Confusing Metric with Administrative Distance:

  • Metric selects the best path within the same routing protocol.
  • Administrative Distance selects the most trusted route between different routing sources.

• Believing default routes replace all routing entries; they are only used when no more specific route exists.

• Assuming all routing protocols support VLSM; classful protocols do not.

Short Exam Notes

• IP Routing: Forwarding packets between different networks.
• Routing Table: Database of reachable networks stored in RAM.
• Static Routing: Manual configuration; suitable for small networks.
• Default Routing: Uses 0.0.0.0/0; ideal for stub routers.
• Dynamic Routing: Automatically exchanges routing information.
• Routed Protocols: IPv4, IPv6, IPX, AppleTalk.
• Routing Protocols: RIP, OSPF, EIGRP, IS-IS, BGP.
• IGP: Used within one organization.
• EGP: Used between autonomous systems (BGP).
• Distance Vector: Uses hop count; periodic updates; examples: RIP, IGRP.
• Link-State: Uses SPF and cost; examples: OSPF, IS-IS.
• Hybrid: Combines DV and LS features; example: EIGRP.
• Classful: No subnet mask in updates (RIPv1, IGRP).
• Classless: Includes subnet mask; supports VLSM (RIPv2, OSPF, EIGRP, IS-IS).
• Administrative Distance: Lower value = more trusted.
• Default AD Values: Connected = 0, Static = 1, EIGRP = 90, OSPF = 110, RIP = 120, Unknown = 255.