Tuesday, 2 July 2013

EIGRP Load Balancing

EIGRP Load Balancing




  • Configure IP addresses on interfaces and loopbacks
  • Configure EIGRP
  • EIGRP topology table
  • View debugging for EIGRP
  • Configure and verify equal-cost load balancing
  • Configure and verfiy unequal-cost load balancing

Configure EIGRP

I'm going to set the serial interfaces clock rate and bandwidth to 64kbps to show normal load balancing.

I have configure EIGRP for ALPHA and GAMMA, I will finish the config off on BETA with debug ip eigrp 1 (1 being the AS number).

The debugging information shows DUAL completeing the topology table (the image does not show all the output).

With EIGRP configured we can now view the routes in the routing table.

You can use this command to view EIGRP neighbours and verify the configuration;

EIGRP topology table

When we view the topology table we can check the Successor route, the Advertised Distance (AD), and the Feasible Distance (FD).

From the output below we can see that both ALPHA ( and GAMMA ( are advertiseing that they can reach the network. They also have the same AD of 40512000 and the same FD of 41024000. Passive means that the route is being used.

We can look further into the routes being advertised from ALPHA and GAMMA with the following command;


Equal-cost load balancing

From the output above you can see that EIGRP is using equal-cost load balancing to the network.

To see the effect of EIGRP equal-cost load balancing we are going to disable Cisco Express Forwarding (CEF). I am only disabling CEF to illustrate load balancing, CEF would not normally be disabled on a network.

From the output, you can see that EIGRP load balances between s0/0 and s0/1.

To see all the routes that BETA has learned via EIGRP, you can use the following command;

We can view alternative paths to the network, which includes paths with higher AD's than the FD;

Unequal-cost load balancing

If we view the metrics advertised by EIGRP for the network, we can see the AD for a loopback is higher than the FD, so DUAL will not consider it a successor route.

To demostrate unequal-cost load balancing I will increase the bandwidth on the path to the destination network through ALPHA.

I have configured the routers with the new speed settings (128 BW and 128000 clock rate).

You can use the following commands to verify your configuration;

From the information you can see the prefered route to is now through OMEGA. If we view the routing table we can see the prefered route is via OMEGA.

Now to allow unequal-cost load balancing we need to enter the variance 2 command on BETA. This command allows unequal-cost load balancing bounded by a maximum distance of (2) x (FD), where FD represents the FD for each route in the routing table.

I'll also enter the debug command so that we can view the changes. It will show that routes are being added after the variance 2 command has been entered.

Now with the newly installed routes we can check the routing table. From where we can see that the unequal-cost routes are now showing up.

If you look at the topology table you will notice that the unequal-cost routes show up, even though they are not considered successor routes (their AD is not less than the FD).

Load balancing over the serial links occurs in blocks of packets. You can see this information using the following command (see traffic share counts).

Now i'm going to check the load balancing from BETA to ALPHA. We can see that it does alternate between s0/0 and s0/1. I did cut a large portion out from the debugging command.



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