Basic Load Balancing

Load Balancing normally has two modes of operation: Per-Destination and Per-Packet.

Per destination load balancing means the router distributes the data packets based on the destination address. If you have two paths going to Host A & B on the same network then all packets for Host A will travel over the first path and all packets for Host B will travel over the second path. This will preserve the packet order which is very useful in certain applications; however, it could result in unequal usage of the network links as bandwidth & load are not taken into route calculation.

Per packet load balancing means the router sends one packet over the first path and second packet over the second path; all going to the same destination. Per packet load balancing guarantees equal load across all the links; however, the packets may arrive at the destination out of order because of different delay/bandwidth may exist on different paths.

Per destination load balancing is enable by the command:

Router# config t
Router(config)# interface Ethernet 0
Router(config-if)# no ip route-cache

Now the router CPU will look at every single packet and spread them across the different path available in the routing table for the destination. This is not recommended on low end server as it could crash the router because the CPU must do all the processing and might not be able to handle it. To enable fast switching, use the following commands:

Router# config t
Router(config)# interface Ethernet 0
Router(config-if)# ip route-cache

Newer switching schemes such as Cisco Express Forwarding (CEF) allow you to do per packet and per destination load balancing more quickly but it does imply that extra resources will be needed to maintain it.

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.