Default Routes
Sections:
Use Case Scenarios:
- A common use case of the default route is to send home network or small office outbound traffic out to an internet facing router or gateway towards the Internet
- Default routes can be used as a fallback route for dynamic routing protocols by injecting the default route into every router in the domain
Overview:
- Default routes are used to direct packets destined for any network that is not explicitly listed in the routing table and acts as a catch all route
- Default routes help manage traffic for unknown destinations by forwarding it to a specified next-hop address or exit interface
- In this lab scenario, I have bridged my GNS3 lab topology to my home network to have access out to the Internet
- In this example, I will configure a default route on R1, R2, and R3 for any networks that have not been explicitly defined in the routing tables on each router
- Up to this point, we know that all routers are going to need route entries for inter-network destinations
- In order for my GNS3 networks to get out to the Internet, my home Internet gateway router will need routing entries that point back to my bridged GNS3 topology networks that are not directly connected
- As I'm using ISP equipment to get out to the Internet from home, I'm unable to add those static route entries to its routing table
- As a work around, I have configured NAT on R1 to modify the source IP address of the packets from the GNS3 networks that are not directly connected to my Internet gateway device
- In a future lesson, we will discuss NAT more in depth as NAT is primarily used to enable multiple devices on a private network to share a single public IP address when accessing the internet
Lab Topology
Configuration
Default Routes: R1
Notes:
- From R1, I have set an IP address on the interface facing my Internet provider's gateway router
- I have then configured the R1's default route using command: 'ip route 0.0.0.0 0.0.0.0 192.168.4.1'
- To summarize, for any other networks the router does not have explicitly defined, send out towards gateway router at IP 192.168.4.1
- I have then issued a ping towards Google's public DNS server at 8.8.8.8 to test Internet outbound traffic
- To verify a default route has been configured you can issue the 'show ip route' command and check to see if their is a 'Gateway of last resort' set
Default Routes: R2
Default Routes: R3
Reachability
Scenario: In this scenario, we will test Internet connectivity on PC1, PC2, and PC3 after configuring the default routes on R1, R2, and R3
Notes:
- From the ping results, PC1 is able to ping Google's Internet facing DNS server at 8.8.8.8
- Using traceroute, PC1 takes the path out to the public next hop routers on the Internet before it gets to the DNS server
