Day 11: Routing Fundamentals

CCNA 200-301 Study Guide: IP Routing Fundamentals & Static Routing

1.0 Introduction: The Core of IP ConnectivityOverview

IPWhat routingis Routing?

Routing is the Layer 3 functionprocess that enablesrouters communicationuse acrossto 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 segments.boundaries)

It

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 majorRouting componentTable?

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

domain,

Stored representingon: 25%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 CCNARouting exam.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:

Layer1. 2 Switching vs. Layer 3Static 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., 192.168.2.00)

Deconstructing the

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

next-hop-ip — IP Routing Table

The routing table is a RAM-based mapaddress of the network.next Therouter command(e.g., 10.0.0.2)

Example 1: Basic Static Route

Scenario:

Router A at 10.0.0.1 needs to viewreach itnetwork 192.168.2.0/24

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

Configuration:

RouterA(config)# ip route.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

RoutingExample Table2: ComponentsMultiple 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:

AutomaticallyExample Populated3: RoutesNext-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:

• ConnectedPC A (C):in The10.0.0.0 networkLAN) segment directly attachedneeds to anreach activePC interface.B (in 192.168.1.0 LAN)

• LocalNeed (L):static Aroutes hoston routeboth ($/32$) for the specific IP assigned to the router's interface.routers

3.0

Configuration Theon PathRouter Selection Process: A Router's LogicA:

RoutersRouterA(config)# follow a non-negotiable, three-step hierarchical logic to determine the "Best Path."

Longest Prefix Match (LPM): The router prefers the most specificip route (the192.168.1.0 one255.255.255.0 with the longest subnet mask).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:

• Example:Router ForA destinationknows 10.how to reach 192.168.1.1.5, a /32 route beats a /0/24 route.

  (sends

to Router

Administrative Distance (AD): If prefix lengths are identical, the router selects the source with the lowest AD.

Connected: 0B)

Static:Router 1B knows how to reach 10.0.0.0/24 (sends to Router A)

OSPF:PC 110A 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:

Metric:Small If both prefix length and AD are identical, the path with the lowest costnetworks (metric)< is10 chosen.routers)

4.0 Static Routing: Manual Network Navigation

StaticSimple, routesstable aretopologies

manually configured using

Point-to-point thelinks

ip route command.

High Theysecurity arerequirements

ideal for "Stub"

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 with(> a10 singlerouters)

exit path.

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

Configuration MethodsVerification

Example Session

Router# configure terminal

Router(config)# ip route 192.168.2.0 255.255.255.0 192.168.1.10.0.0.2

Router(config)#

Exit Interface: Primarily for point-to-point links.

ip route 10.2.2.192.168.3.0 255.255.255.0 g0/110.0.0.2

Router(config)#

Fully Specified: Provides both interface and next-hop; avoids recursive lookups.

ip route 10.2.2.0 255.255.255.0 g0/1 192.168.1.2

5.0 The Default Route: Gateway of Last Resort

The default route acts as a safety net for packets that do not match any other specific entry in the routing table.

Syntax: 0.0.0.0 0.0.0.0 (CIDR notation: /0).203.0.113.1

Router(config)#

exit

Logic:Router# Becauseshow 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 hasfor aforwarding prefixdecisions

length

✅ ofStatic zero,routing it= ismanual theconfiguration; leastgood 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 possiblewins andwhen ismultiple onlyroutes used if all other entries fail to match.

Verification: Identified in the routing table by the code S*. The asterisk (*) signifies the active candidate for the gateway of last resort.

6.0 Packet Forwarding Mechanics: L2 and L3 Journey

Asmatch a packet travels across routers, its encapsulation changes.

IP Addresses (Layer 3): Remain unchanged from source to destination (unlesslongest NATprefix is applied).match)

✅

Router must have routes in both directions for bidirectional communication

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

✅ Local routes (LayerL) 2): Are rewritten at every hop. Each router replacesare the source MAC with itsrouter's own exitIP interfaceaddresses; andhandled the destination MAC with the next hop's address.automatically

The Forwarding Procedure

De-encapsulate: Strip the L2 frame to see the L3 packet.

Lookup: Find the best path in the routing table.

Re-encapsulate: Wrap the packet in a new L2 frame for the next hop.

7.0 Core Troubleshooting for Static Routes

Verify Interface Status: Use✅ show ip interface brief. Interfaces must be up/up. A static route isdisplays removedrouting iftable; itsincludes exitsource interfaceand ismetric down.information

✅

Routers

Confirmseparate Routebroadcast Presence:domains Use— showbroadcasts ipdon't route. Ensure no typos were made during configuration.

Recursive Lookup Check: Thecross router must have a valid route to the Next-Hop IP for the static route to be installed in the table.interfaces

Ensure Route Symmetry: Check that the return path exists on the remote router. Routing is a one-way decision.

8.0 Summary of Key Concepts

LPM is the first rule of routing; specificity always wins.

Static Routes have a default AD of 1.

Connected Routes have the lowest possible AD of 0.

Default Routes handle all otherwise unmatchable traffic and use the /0 mask.