Add Redundancy With Backup Command

You can add redundancy to your network and make you network more resilient with the Backup command. What this command does is specify an interface which will act as a back up in case the primary interface fails.

Let's take a look at the following configurations:

On RouterA:

Configure terminal
Interface f0/0
Ip address 192.168.1.1 255.255.255.0
no shutdown
Interface f0/1
Ip address 192.168.2.1 255.255.255.0
no shutdown
Backup interface f0/0
Exit
Router eigrp 20
network 192.168.1.0
network 192.168.2.0

On RouterB:

Configure terminal
Interface f0/0
Ip address 192.168.2.2 255.255.255.0
no shutdown
Interface f0/1
Ip address 192.168.2.2 255.255.255.0
no shutdown
Backup interface f0/0
Interface loop 0
Ip add 192.168.8.1 255.255.255.0
Exit
Router eigrp 20
network 192.168.1.0
network 192.168.2.0
network 192.168.8.0

With these configurations loaded, both Fa0/1's will change to up/up while Fa0/0's will go into standby mode and monitor the activities on Fa0/1. In the event that Fa0/1 goes down, Fa0/0 will switch to active mode and establish neighbor relationship with the connected interface and complete the failover.

To verify this, from RouterA you can ping 192.168.8.1 with the repeat parameter:

Ping 192.168.8.1 repeat 10000

This command will ping 192.168.8.1 10k times. While it's pinging, unplug the cable to Fa0/1. you will see that the link go down, the ping will fail and then within seconds Fa0/0 will be up and resume the active role and the pinging will resume as nothing has happened. If the link on Fa0/1 becomes active again, Fa0/0 will switch back to standby mode.

This is a quick and easy way to add redundancy to your network. There are other techniques that you can use to add redundancy to your network to include both layer 2 and layer 3 redundancies which I will discuss at another opportunity.