EIGRP – Part 1

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EIGRP is my favorite IGP (don’t hate me just yet, I have a good reason).  I’ll elaborate more on that comment later, but for now, let’s jump right into a topology so we can start playing with EIGRP…


In my lab, I’m using 1841 routers and since they only have two Ethernet interfaces, I’m going to be utilizing sub interfaces in this example.  The initial config involves configuring the interfaces on each router and ends with each router being able to ping it’s directly connected neighbor. I’m not going to run through that config since it’s pretty straight forward.

The bottom router (Router5) has a /24 network behind it which we want to be reachable by all the routers.  The top switch (Switch1) has the 0’s route behind it which we also want to be reachable by all of the routers.  Let’s put a base configuration on each router to get EIGRP up and running and see what it does.  The base configuration will look like this…

<DEVICE>#config t
Enter configuration commands, one per line.  End with CNTL/Z.
router eigrp 1
<DEVICE>(config-router)#network <Physical Interface IP>

So for instance, the switch configuration would look like this…


Once, that’s done on all of the routers, let’s look at the routing table on router5…


You’ll note that we have two routes that have dual paths to the destination prefix.  Those are the prefix as well as the prefix.  Looking at the topology again, this makes sense since the paths to each of those prefixes from router 5 would truly be equal cost. 

No other real surprises here.  The rest of the /30’s are listed in the routing table as we would expect since all of the routers successfully joined EIGRP.  Let’s talk a quick second to talk about route selection.  As you know (or should know, read this if you don’t) EIGRP has an admin distance of 90.  Any route to the same destination with a higher admin distance will get squashed by the EIGRP route.  The admin distance of 90 is for what are called ‘internal’ EIGRP routes.  There are also ‘external’ EIGRP routes that have an admin distance of 170.  External EIGRP routes would be routes leaked into a EIGRP AS from a different EIGRP AS through route distribution.

However, what we don’t know is how EIGRP decides which route it is going to pick to get to each prefix.  For instance, we see above that to get to router5 had two paths to the destination.  That’s because these two paths are equal cost so EIGRP can safely insert them both into the FIB and let CEF handle the load balancing between the two next hops.  However, since router5 is dual homed, it does have two paths to get to each and everyone of the other prefixes as well.  So where are those?

Before we go too far down the rabbit hole, let’s define some EIGRP terminology that should help us stay on track…

The goal of every EIGRP router is to form an adjacency with a neighboring EIGRP router.  Once this adjacency has been formed, the router will receives EIGRP updates from it’s peer router.  The updates can then be processed ,along with the metric to get to the peering router, to determine a distance to the prefix learned in the advertisement.

Feasible Distance (FD)
Of all of the paths learned to each given prefix, one will be picked as the ‘best’ path.  This metric associated with this best path is known as the ‘feasible distance’.  Basically, the router just picks the lowest cost path and makes that the FD. 

Advertised Distance (AD)
The advertised distance is the metric from the neighbor advertising the prefix, to the prefix.  That is, the feasible distance minus the cost to get to the neighbor you heard the advertisement from. 

Feasibility Condition (FC)
The feasibility condition states that a route may become a feasible successor route if the advertised distance is less than the feasible distance. 

A successor is the best route to a prefix

Feasible Successor
A feasible successor is a route that matches the feasibility condition and can be used as a backup route. 

Unfortunately, we need to go a little but further down the rabbit hole to fully understand EIGRP. To do that, we need to understand the metric that EIGRP uses. The metric itself is rather complicated, but if you assume the use of default values, one can size the formula down to this…


Let’s take a look at our example to clarify things.  So here is the output of the EIGRP topology table on router5…


Using that formula, let’s make sure we understand where the metric are coming from that EIGRP is reporting.  For instance, let’s examine the prefix /29.  Note that the EIGRP topology table is showing 2 successors for the prefix, with two next hops that have matching metrics.  If the metrics weren’t a dead match for each other, we wouldn’t have 2 successors (in most cases, more on that later).  The topology table is reporting a feasible distance of 33280 and a advertised distance of 30720 for both paths to that prefix.  Let’s break down the math to see where that comes from.

Recall, we really only need to come up with two variables to solve the equation, the minimum bandwidth along the entire path and the sum of all interface delays.  Our lab topology currently uses fast Ethernet interfaces with the default delay and speed configuration, so the path looks something like this…


The information on the delay as well as the interface speed can be shown using the ‘show interface <interface>’ command.  Here we see the speed and delay of an interface on router5…


Note: The delay can be statically set on an interface (we’ll do that later).  However, when you set the delay you are setting it in ‘tens of microseconds’ which are not the same as the usec metric.  If you set the delay to 50 on an interface, it will be 500 usec.  It’s good practice to verify what was set with the show interface command to make sure it’s what you think you configured. 

So, we can see here the the minimum bandwidth is 100 meg (100000 Kb) on any of the given links.  So there is out first variable, our second variable is the sum of the interface delays which we can see would equal 300 usecs.  Plug those into the formula…


and we get a metric of 33280.  So that’s our feasible distance, how about our advertised distance?  Recall that the advertised distance is the metric a router hears advertised by it’s neighbor not including the cost to get to the neighbor.  In this case, that would mean that we’d just remove the delay of router5’s interface and run the equation again…


Which gives us an advertised distance of 30720 like we saw in the output above. 

Now, let’s look at a prefix that only has one entry in the topology table.  For instance, let’s look at the prefix /30.  The EIGRP topology table shows only one path to that prefix…


And why is that? The answer is simple, all other paths must fail the feasibility condition.  Let’s look at the other paths to prove this…

The path highlighted in orange is obviously the shortest path to the prefix, but let’s look at the purple path to prove why it can’t be a feasible successor.   The metric for the orange path is 30720 which is the prefixes feasible distance.  Let’s compute the metric for the purple path.  The minimum bandwidth on that path is 100000 Kbps but the delay is 400 usec.  That would gives us a metric of 35840 for the purple path.  The feasibility condition states that a route may become a feasible successor route if the advertised distance is less than the feasible distance. Since 35840 is not less than 30720, the route is not a feasible successor to the existing successor.  Let’s shut down the link between router3 and router5 to prove that our metric calculation was valid…


I show the use of a very handy command here.  The ‘show ip eigrp topology <prefix>’ command is very handy in determining the metric and the means in which it was calculated.  As you can see, we started wit the FD of 30720 (orange path) and after shutting down the interface, we came up with a FD of 35480 (purple path). 

Hopefully, at this point, EIGRP is starting to make a little more sense to you.  To mix things up a bit (and show us some other features) let’s manipulate some of the interfaces to change things up a little bit…

Notice that we’ve manipulated the delay on the links between router1, router3, and router5.  A look at the topology table on router5 now shows us…


2 prefixes to the /30 subnet.  Let’s do the math on each of these to figure out how this happened. 

The Orange Path Feasible Distance

The Purple Path Feasible Distance

The winner in this case turns out to be the purple path since it has a lower metric.  However, the advertised distance of the orange path turns out to be 281600 which happens to be less than the feasible distance of the purple path.  When this happens the route becomes a feasible successor.  The output of the topology table confirms this for us…


Taking a close look at the topology table, we can tell that we are dealing with a feasible successor…


Note that the table says it has 1 successor, but there are two entries.  Only the first entry will show up in the forwarding table of the router…


Since there is a second entry, we know that EIGRP is using the second route as a feasible successor.  If EIGRP is going to insert both routes into the routers routing table, it will tell you that you have 2 successors as we saw above with the /29 prefix…

Topology Table

Router’s routing Table

I think that’s enough for now.  There are a few more aspects of EIGRP I’d like to cover in my next post but I think this is enough to get started.  Look for post 2 coming up next!

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