Default Route

 Default Route 

When the router receives a packet for a destination that does not match any more specific route in its routing table then we send it to default gateway.. 

Network Topology

              
                R2 (ISP)
         G0/0 203.0.113.2/30
                    |
          G0/1 203.0.113.1/30
                   R1 
          G0/0 192.168.10.1/24
                    |
                 Switch
                    |
                   PC1
            192.168.10.10/24

IP Addressing

DeviceInterfaceIP Address
PC1NIC192.168.10.10/24
R1G0/0192.168.10.1/24
R1G0/1203.0.113.1/30
R2G0/0203.0.113.2/30

PC Default Gateway:

192.168.10.1

Step 1: Configure R1

enable
configure terminal

interface g0/0
ip address 192.168.10.1 255.255.255.0
no shutdown

interface g0/1
ip address 203.0.113.1 255.255.255.252
no shutdown

Step 2: Configure R2

enable
configure terminal

interface g0/0
ip address 203.0.113.2 255.255.255.252
no shutdown

Step 3: Configure the Default Route on R1

ip route 0.0.0.0 0.0.0.0 203.0.113.2

Syntax

ip route 0.0.0.0 0.0.0.0 < Exit interface / ISP Gateway >

This means:

"If I don't know how to reach a destination, send the packet to 203.0.113.2."

Verify the Route

show ip route

Example output:

Gateway of last resort is 203.0.113.2

S* 0.0.0.0/0 [1/0] via 203.0.113.2

C 192.168.10.0/24 is directly connected

C 203.0.113.0/30 is directly connected

Explanation

  • S = Static Route
  • * = Candidate Default Route
  • Gateway of last resort = The default route

Test Connectivity

From the PC:

ping 203.0.113.2

Then test a destination not in the local routing table:

ping 18.8.8.8

The packet flow is:

PC
 ↓
R1
 ↓
Default Route
 ↓
R2
 ↓
Internet

Using the Exit Interface Instead of a Next Hop

You can also configure:

ip route 0.0.0.0 0.0.0.0 g0/1

This tells R1 to send unknown traffic out interface G0/1.

Remove the Default Route

no ip route 0.0.0.0 0.0.0.0 203.0.113.2

Troubleshooting Commands

show ip route

show running-config

show ip interface brief

show arp

show cdp neighbors

ping 203.0.113.2

traceroute 8.8.8.8


Think of it like this


    

  • Default gateways = doors inside a building (one door for each department/VLAN).

    • 

  • Public IP = the building's single main street address.

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