Day 11: Routing Fundamentals

Routing Fundamentals & Static Routing

Overview

What is Routing?

Routing is the process that routers use to determine the path that IP packets should take over a network to reach their destination.

Core Concept:

• Routers forward packets between different networks (across network boundaries)

• Switches forward frames within the same LAN (within a network)

• A router reads the destination IP address and determines where to send the packet next

Key Difference from Switching:

Switches (Layer 2):

- Examine destination MAC address

- Forward within the same LAN

- Fast forwarding to connected devices

Routers (Layer 3):

- Examine destination IP address

- Forward between different networks

- Use routing tables to determine best path

 

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Routing Tables

What is a Routing Table?

A routing table is a database of routes that tells the router where to send packets destined for specific networks or IP addresses.

Stored on: Every router in the network

Content: Network destinations and how to reach them

Route Components

Each route entry contains:

Destination Network → Next-Hop Router (or directly connected)

Example:

Destination: 192.168.2.0/24 → Send to next-hop router: 10.0.0.2

Destination: 10.1.1.0/24 → Send directly out interface G0/1

Destination: 172.16.0.0/16 → Send to next-hop router: 10.0.0.3

Route Types

When a router receives a packet, it can take one of three actions:

1. Forward to Next-Hop Router

Route: "To reach 192.168.1.0/24, send to router 10.0.0.2"

Router receives packet destined for 192.168.1.50

↓

Looks in routing table: "I have a route to 192.168.1.0/24"

↓

Action: Forward packet to 10.0.0.2 (next-hop IP)

↓

Router 10.0.0.2 receives and continues forwarding

2. Forward Directly to Destination (Directly Connected)

Route: "192.168.1.0/24 is directly connected to my G0/1 interface"

Router receives packet destined for 192.168.1.100

↓

Looks in routing table: "192.168.1.0/24 is on my G0/1 interface"

↓

Action: Send packet directly out G0/1 to destination

3. Receive for Itself

Route: "10.0.0.0/24 is my own interface"

Router receives packet destined for 10.0.0.1 (its own IP)

↓

Looks in routing table: "This is MY IP address"

↓

Action: Don't forward, receive packet and process it

Viewing the Routing Table

Router# show ip route

This displays all routes the router knows about.

Output includes:

Gateway of last resort is not set

    10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C       10.0.0.0/24 is directly connected, GigabitEthernet0/0

L       10.0.0.1/32 is directly connected, GigabitEthernet0/0

    192.168.0.0/16 is variably subnetted, 2 subnets, 2 masks

C       192.168.1.0/24 is directly connected, GigabitEthernet0/1

L       192.168.1.254/32 is directly connected, GigabitEthernet0/1

S       192.168.2.0/24 [1/0] via 10.0.0.2

Legend:

C = Connected (directly connected)

L = Local (router's own IP)

S = Static (manually configured)

D = Dynamic (learned from routing protocol)

R = RIP, O = OSPF, B = BGP (dynamic protocols)

 

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Routing Methods

There are two main ways routers learn routes and populate their routing tables:

1. Static Routing

What it is:

• Network engineer/administrator manually configures routes

• Routes do NOT change automatically

• Engineer must manually update routes if network changes

Advantages:

• Simple to understand and implement

• No overhead (no routing protocol traffic)

• Secure (no routing information shared automatically)

• Good for small networks

Disadvantages:

• Must manually configure every route

• Doesn't adapt if network fails or changes

• Not scalable for large networks

• Time-consuming to maintain

When to use:

• Small networks (< 10 routers)

• Point-to-point links (few destinations)

• Labs and testing

• When security requires manual control

2. Dynamic Routing

What it is:

• Routers automatically share routing information with each other

• Routers learn and update routes automatically

• Routes adapt when network conditions change

Advantages:

• Automatic route discovery and updates

• Adapts to network failures

• Scalable for large networks

• Less manual configuration

• Load balancing across multiple paths

Disadvantages:

• More complex to configure and understand

• Uses network bandwidth and CPU (routing protocol overhead)

• Slower initial convergence

• Security concerns (routing information shared)

Common Dynamic Routing Protocols:

When to use:

• Medium to large networks

• Complex topologies

• Multiple paths to destinations

• Networks with redundancy

• When automatic failover is needed

 

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WANs (Wide Area Networks)

What is a WAN?

A Wide Area Network (WAN) is a network that extends over a large geographic area — typically connecting multiple locations (cities, countries, or across the Internet).

Characteristics:

• Connects different LANs together

• Uses routers to connect

• Often uses leased lines or Internet as backbone

• Lower bandwidth and higher latency than LANs

• Can span great distances

Example WAN Architecture:

       [LAN A]

         |

      Router A ←→ Router B ← WAN Link (Internet or Leased Line)

         |          |

      [LAN B]    [LAN C]

Routers A and B are connected via WAN

Each router serves its local LAN

Users in LAN A can reach LAN B/C through routers

LAN vs WAN

 

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Static Routing Configuration

Basic Static Route Syntax

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

Components:

• destination-network — The network you want to reach (e.g., 192.168.2.0)

• subnet-mask — Netmask of that network (e.g., 255.255.255.0)

• next-hop-ip — IP address of the next router (e.g., 10.0.0.2)

Example 1: Basic Static Route

Scenario:

• Router A at 10.0.0.1 needs to reach network 192.168.2.0/24

• Router B (next-hop) is at 10.0.0.2 on a shared link with Router A

Configuration:

RouterA(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2

Translation: "To reach 192.168.2.0/24, send packets to my neighbor at 10.0.0.2"

Result in routing table:

S       192.168.2.0/24 [1/0] via 10.0.0.2

Example 2: Multiple Static Routes

Scenario:

Router A needs to reach two different networks:

RouterA(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2

RouterA(config)# ip route 172.16.0.0 255.255.0.0 10.0.0.3

Result:

• Packets destined for 192.168.2.x go to 10.0.0.2

• Packets destined for 172.16.x.x go to 10.0.0.3

Static Route Configuration with Exit Interface

Alternative Syntax (for point-to-point links):

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

Example:

RouterA(config)# ip route 192.168.2.0 255.255.255.0 Serial0/0/0

Translation: "To reach 192.168.2.0/24, send out my Serial0/0/0 interface"

When to use:

• Point-to-point WAN links (Serial, PPP)

• When only one neighbor on the link

• More efficient on point-to-point connections

Difference:

Example 3: Next-Hop vs Exit Interface

Next-Hop (for Ethernet):

RouterA(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2

                                                    ↑

                                            specific neighbor IP

Exit Interface (for Serial):

RouterA(config)# ip route 192.168.2.0 255.255.255.0 Serial0/0/0

                                                    ↑

                                              output interface

 

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Default Route

What is a Default Route?

A default route is a "catch-all" route that handles packets destined for networks the router doesn't have specific routes for.

Use: Send unknown destinations to a central router or ISP gateway

Default Route Configuration

Syntax:

Router(config)# ip route 0.0.0.0 0.0.0.0 <next-hop-ip>

Or in CIDR notation:

Router(config)# ip route 0.0.0.0/0 <next-hop-ip>

Translation: "For any destination I don't have a more specific route for, send to this router"

Common notation: The default route is often written as 0.0.0.0/0 (the "default route")

Example: Setting a Default Route

Scenario:

Small office network with single ISP connection

Router(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1

Translation: "For any destination not in my routing table, send to the ISP gateway at 203.0.113.1"

Result:

S*   0.0.0.0/0 [1/0] via 203.0.113.1

     *  indicates this is the default route

Route Precedence (Most Specific Wins)

When multiple routes match a destination, the router uses the most specific route:

Routing table:

0.0.0.0/0 → 203.0.113.1 (default route)

192.168.2.0/24 → 10.0.0.2 (specific route)

192.168.2.0/25 → 10.0.0.3 (more specific)

Packet destined for:

192.168.2.10 → Uses 192.168.2.0/25 (longest prefix match — /25 most specific)

192.168.3.10 → Uses 0.0.0.0/0 (default — no other match)

172.16.0.1 → Uses 0.0.0.0/0 (default — no other match)

 

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Static Routing Configuration Example

Complete Scenario

Network setup:

         [PC A]

            |

        [Switch]

            |

        Router A ────WAN Link──── Router B ────[Switch]──── [PC B]

       10.0.0.1    (Serial)      10.0.0.2                192.168.1.0/24

      

Router A is in: 10.0.0.0/24

Router B is in: 10.0.0.0/24 and 192.168.1.0/24

Requirements:

• PC A (in 10.0.0.0 LAN) needs to reach PC B (in 192.168.1.0 LAN)

• Need static routes on both routers

Configuration on Router A:

RouterA(config)# ip route 192.168.1.0 255.255.255.0 10.0.0.2

RouterA(config)# end

RouterA# show ip route

S       192.168.1.0/24 [1/0] via 10.0.0.2

Configuration on Router B:

RouterB(config)# ip route 10.0.0.0 255.255.255.0 10.0.0.1

RouterB(config)# end

RouterB# show ip route

S       10.0.0.0/24 [1/0] via 10.0.0.1

Result:

• Router A knows how to reach 192.168.1.0/24 (sends to Router B)

• Router B knows how to reach 10.0.0.0/24 (sends to Router A)

• PC A can now ping and communicate with PC B

Verifying Routes

Check routing table:

Router# show ip route

Test connectivity:

Router# ping 192.168.1.100

Trace packet path:

Router# traceroute 192.168.1.100

 

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Comparison: Static vs Dynamic Routing

When to Use Static Routing

✅ Good for:

• Small networks (< 10 routers)

• Simple, stable topologies

• Point-to-point links

• High security requirements

• Lab/testing environments

• When you want complete control

❌ Not good for:

• Large networks

• Complex topologies

• Frequent topology changes

• Networks requiring redundancy

• Automatic failover

When to Use Dynamic Routing

✅ Good for:

• Large networks (> 10 routers)

• Complex topologies

• Multiple paths to destinations

• Automatic redundancy/failover

• Networks that change frequently

• Production enterprise networks

❌ Not good for:

• Very small networks (overhead not worth it)

• When bandwidth is extremely limited

• When security isolation is critical

• Simple point-to-point links

 

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Static Route Commands Reference

Configuration

Verification

Example Session

Router# configure terminal

Router(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2

Router(config)# ip route 192.168.3.0 255.255.255.0 10.0.0.2

Router(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1

Router(config)# exit

Router# show ip route

    10.0.0.0/24 is directly connected, GigabitEthernet0/0

S   192.168.2.0/24 [1/0] via 10.0.0.2

S   192.168.3.0/24 [1/0] via 10.0.0.2

S*  0.0.0.0/0 [1/0] via 203.0.113.1

Router# ping 192.168.2.100

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echoes to 192.168.2.100, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/5 ms

 

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Key Differences: Switches vs Routers in Network

 

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Key Takeaways for CCNA

✅ Routing is how packets move between different networks using destination IP addresses

✅ Routing table stores all known routes; router uses it for forwarding decisions

✅ Static routing = manual configuration; good for small, stable networks

✅ Dynamic routing = automatic discovery; good for large, complex networks

✅ WAN connects separate LANs over large geographic distances

✅ Default route (0.0.0.0/0) catches packets with no other matching route

✅ Static route syntax: ip route <destination> <mask> <next-hop-ip>

✅ Exit interface method used for point-to-point links; next-hop method for multi-access

✅ Routes point to next-hop IP or exit interface — tells router where to send packets

✅ Most specific route wins when multiple routes match a destination (longest prefix match)

✅ Router must have routes in both directions for bidirectional communication

✅ Directly connected routes appear as 'C' in routing table; don't need to be configured

✅ Local routes (L) are the router's own IP addresses; handled automatically

✅ show ip route displays routing table; includes source and metric information

✅ Routers separate broadcast domains — broadcasts don't cross router interfaces