Identifying How EIGRP Handles Traffic
EIGRP uses minimum bandwidth, and cumulative delay of a route by default to determine its metric. As such we can go into the routers and manipulate these settings to adjust how EIGRP sends traffic. We're also going to look at load balancing and
We know from router 1's topology table on the previous page that when routing to router 5 it will go through router 2. It does this because we already changed the bandwidth setting on s0/1 to be lower than s0/0. Now we're going to make a few changes to router 1 and see how it handles traffic. The first thing I'll do is issue the 'no bandwidth' command to take off the custom bandwidth on interface s0/1. This will make s0/1 and s0/0 the same bandwidth and router 1 will begin routing through router 3 when heading to router 5. Now I will make the following changes -
How do you think EIGRP will route traffic to router 5 now? Well let's do a traceroute and find out -
We see that router 1 went through router 3. Even though the s0/0 interface has a higher bandwidth setting than the s0/1 on router 1 EIGRP uses the lowest bandwidth of the entire path for its calculation. Since router 2's s0/1 interface also has a 10Mbps bandwidth EIGRP didn't use that link. Let's change the bandwidth of router 1's s0/0 interface down to 10Mbps so that all the point to point interfaces are equal and then alter the delay according to the following graphic -
We went through how bandwidth can alter the path router 1 takes to router 5, now let's try to figure out how delay will impact this decision. Well if we do a traceroute we find -
Router 1 now goes through router 2, and 4 to get to router 5. It does this because to total delay to get to router 5 through router 2 is 10, while the delay through router 3 is 12. Configuring delay of an interface is the same as bandwidth, simply go into the interface and issue the 'delay' command -
Now that we know how to manipulate traffic with EIGRP lets take a look at load balancing. By default EIGRP will load balance across links with an equal metric. We can change something called a variance to alter how EIGRP load balances across links. EIGRP's default variance is 1, which means it will only load balance across equal cost paths. If we changed the variance to 2 EIGRP would load balance across a path that is half as good as the successor route. Let's take a look at our network currently and then alter the variance.
We see currently 1 entry for the 172.16.0.0/25 network. Now let's go into EIGRP configuration and change the variance to 2.
We see that after we enter the 'variance 2' command in EIGRP config that the routing table added an extra route to several of the entries. EIGRP is now load balancing across these interfaces. Variance can be configured with a value up to 128.