EIGRP III: Choosing the Winner of the Miss EIGRP Pageant!

The Winner! (Feasible Successor)

Even though EIGRP is a simpler protocol in many ways, there are still important concepts to understand, one of which how it chooses routes.  As we talked about before, the lowest metric (constrained bandwidth + cumulative delay) wins the contest for the best route, without the somewhat confusing exceptions that exist in OSPF.  However, because there are elements of Distance Vector routing in this hybrid protocol, there inevitably have to be some kind of loop prevention mechanisms.

EIGRP uses two forms of the calculated metric to a network to select the best loop-free route, and at first glance they might sound identical.  The first value is called the Feasible Distance (FD), which is the complete metric from the router to the destination network.  The second value is called the Reported Distance (RD), which is the metric from the point of view of the next-hop router.  If a path is loop-free, the value of the RD will be less than the FD, expressed mathematically as RD < FD.  Paths that could cause loops will have opposite values, which will make EIGRP discard it.  That may sound simple enough, but as you have probably heard on infomercials, “But wait, there’s MORE“!

Routes that meet the RD < FD test (called the Feasible Condition) are held in the EIGRP Topology Table and the best route gets installed in the IP Routing Table.  This best route is called the Successor, or Successor Route in EIGRP, and if other equal-cost routes exist they are also flagged as successors and installed in the routing table.  One of the truly amazing features of EIGRP, however, has to do with backup routes.  In just about every other protocol, if the primary route fails, the entire convergence process for that failed route starts over to select a new one.  In EIGRP, though, another loop-free path is held in reserve for immediate use if the successor fails; this route is called the Feasible Successor.

If you have ever watched any type of beauty pageant (Miss America, Miss Universe, etc.) then you have actually seen this in action!  After all of the contestants have proceeded through the judging process, one of them is crowned the winner (Successor)!  However, the next contest in line, usually called the first runner-up (Feasible Successor) automatically becomes the winner if anything happens to the original victor–no new pageant is required!

That covers the basic principles of IP routing, next time we will start considering Wide Area Networks!

– Joe

2 Responses to “EIGRP III: Choosing the Winner of the Miss EIGRP Pageant!”

  1. Sir, you have very beautifully explained the concept, I really liked reading it.

    Thank you 🙂

  2. Dear Sir,

    I am preparing for my CCNP certification. Currently preparing BGP.

    First time I am seeing this wonderful Cisco technology blog. Thanks for spending your time to educate others. Will recommend your blog to my friends…

    Tom.

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