Understanding EIGRP Part 6 (Router ID and Neighborship Requirements)

ccie r/s ccna r/s ccnp r/s Feb 23, 2017

This blog post wraps up our series on Understanding EIGRP by discussing two final topics:

  • The EIGRP Router ID
  • EIGRP's Neighborship Requirements

Let's begin our discussion by considering the EIGRP router ID.


Each EIGRP-speaking router has an associated EIGRP router ID (RID). The RID is a 32-bit value written in dotted decimal format, like an IPv4 address. A router’s EIGRP RID is determined when the EIGRP process starts. Interestingly, EIGRP uses the same steps to RID calculation as does OSPF. The following list identifies these step, in sequential order:

Step 1. Use the configured RID value (using the eigrp router-id rid EIGRP router configuration mode command).

Step 2. If no RID is configured, use the highest IPv4 address on a loopback interface in the up/up state.

Step 3. If no loopback interface is configured with an IPv4 address, use the highest IPv4 address on a non-loopback interface.

Interestingly, while EIGRP requires a router to have a RID, the...

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Understanding EIGRP – Part 5 (Static Neighbors)

ccie r/s ccna r/s ccnp r/s Feb 05, 2017

Typically, an EIGRP-speaking router dynamically discovers its neighbors, by sending multicast Hello messages. However, there is an option to statically configure those neighbors, and communicate with them via unicast messages. This is rarely done, but could on rare occasion be useful.

Consider for example a Frame Relay WAN. Imagine that router A has an interface configured with ten Frame Relay permanent virtual circuits (PVCs). At the other end of two of those PVCs resides EIGRP-speaking routers. However, the other eight PVCs do not have an EIGRP-speaking router at the far end. In such a topology, if router A’s WAN interface was participating in EIGRP, then router A would have to replicate its EIGRP Hello message and send a copy out all ten PVCs, resulting in an increased processor burden on router A and increased the bandwidth usage (unnecessarily) on the eight PVCs not connecting to an EIGRP router. This is the type of situation that would benefit from our statically...

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Fundamentals of the Internet of Things (IoT)

It's another one of those buzzwords we're hearing a ton these days, the Internet of Things, or IoT for short.

But what exactly is it, and how's it going to impact us as networking professionals? That's what you'll learn in this new video: 

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Understanding EIGRP – Part 4 (Passive Interfaces)

ccie r/s ccna r/s ccnp r/s Jan 31, 2017

Sometimes, we might want a router interface to participate in an EIGRP routing process (in order to advertise that interface's network) without that interface sending out EIGRP Hello messages. That's what we'll cover in this blog post.

By the way, this is the fourth posting in a series on Understanding EIGRP. If you missed any of the earlier postings, you can check them out here:

Previously, we talked about the network net-id wildcard-mask command issued in EIGRP router configuration mode. This command causes two primary actions:

  1. Sends EIGRP Hello multicast messages out any interface whose IP address falls within the network address space specified by the network command.
  2. Advertises the subnet of any interface whose IP address falls within the network address space specified by the network command....
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Intermediate System to Intermediate System (IS-IS) Routing Protocol Fundamentals

ccie r/s ccna r/s ccnp r/s Jan 20, 2017

In our Cisco routing and switching studies, we commonly study routing protocols such as RIP, OSPF, EIGRP, and BGP. However, there's a very scalable, fast converging, link-state routing protocol that often gets overlooked and forgotten. It's Intermediate System to Intermediate System (or IS-IS for short).

IS-IS is primarily found in service provider environments, but even if you're not in the service provider world, you still might run into it at some point during your career. So, I wanted to create a video to take away the fear, uncertainty, and doubt from IS-IS. In this video, we'll look at the basic theory surrounding IS-IS and then go through a simple configuration.

 Enjoy the video!

Kevin Wallace, CCIEx2 (R/S and Collaboration) #7945

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Understanding EIGRP – Part 3 (EIGRP Timers)

ccie r/s ccna r/s ccnp r/s Jan 09, 2017

Once of EIGRP’s claims to fame is its fast convergence in the event of a link failure. However, one thing that might slow down this convergence is timer configuration. That's the focus of this blog post, which is the third in a series of posts on Understanding EIGRP. If you missed the first couple of posts, you can get them here:

Let's beging our discussion of EIGRP timers by considering a situation where two EIGRP neighbors are directly connected to one another. If the physical link between them fails, each router’s connected interface goes down, and EIGRP can fail over to a backup path (that is, a feasible successor route). Such a situation is shown in the following figure:


Routers R1 and R2, shown in the above figure, are directly attached to one another. Therefore, if the cable between them physically breaks, each of the router interfaces connecting to that link go down, and EIGRP realizes that it...

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Understanding EIGRP – Part 2

ccie r/s ccna r/s ccnp r/s Dec 02, 2016

In the first blog post in our Understanding EIGRP series, we were introduced to EIGRP’s features, in addition to a basic configuration example, and a collection of verification commands. Now, in this post, we’ll delve into the behind the scenes action of how EIGRP establishes a neighborship, learns a route to a network, determines what it considers to be the best route to that network, and attempts to inject that route into a router’s IP routing table.

EIGRP’s operations can be conceptually simplified into three basic steps:

Step 1. Neighbor Discovery: Through the exchange of Hello messages, EIGRP-speaking routers discover one another, compare parameters (for example, autonomous system numbers, K-values, and network addresses), and determine if they should form a neighborship.

Step 2. Topology Exchange: If neighboring EIGRP routers decide to form a neighborship, they exchange their full topology tables with each other. However, after the...

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Understanding EIGRP – Part 1

ccie r/s ccna r/s ccnp r/s Nov 18, 2016

I used to work as a Network Design Specialist at Walt Disney World, in Florida. Their massive network contained over 500 Cisco routers (and thousands of Cisco Catalyst switches). What was the routing protocol keeping all of these routers in agreement about available routes? It was Enhanced Interior Gateway Routing Protocol (EIGRP). That’s the focus of this blog post, which is the first of a series of posts focusing on EIGRP.

If you already have your CCNA R/S certification (or higher), you’re probably well acquainted with EIGRP. However, unless you have an eidetic memory (like Sheldon Cooper on The Big Bang Theory TV show), you probably don’t remember every single EIGRP command and concept. Therefore, this series of blog posts is going to review and reinforce those fundamental EIGRP concepts, and even introduce a few other fun facts.

EIGRP Fundamentals

There’s a long-running debate about the fundamental nature of EIGRP. At its essence, is EIGRP a link...

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Your Framework for Cisco Exam Preparation

You want to get your first (or next) Cisco certification, but do you have a specific preparation strategy, or are you just winging it? If you do have a structured framework you’re confidently executing against, congratulations! If not, allow me to share my seven-step framework for Cisco exam preparation.

Step #1: Identify Your Training Source

When it comes to learning what you need to learn, the good news is, you’ve got options. Let’s compare a few:

Cisco Learning Partner

The traditional approach, and the way I trained certification candidates for nearly fourteen years, is to take an official Cisco course from a Cisco Learning Partner (CLP). You might have the option of taking your course at a training facility or on-line, and you typically get access to the gear you need to perform lab tasks during the course. You also get Cisco’s official course material. However, a gotcha that many people don’t realize is that Cisco typically has different groups...

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BPDUGuard: A Spanning Tree Protocol Enhancement

ccie r/s ccna r/s ccnp r/s Aug 30, 2016

Yet another new topic on the new CCNA R/S v3 exam is BPDUGuard, which is an enhancement to Spanning Tree Protocol (STP) available on our Cisco Catalyst switches. Specifically, BPDUGuard can help prevent a Layer 2 topological loop by placing a port configured for PortFast into an Error-Disabled state if that port receives a Bridge Protocol Data Unit (BPDU).

This video demonstrates the operation of BPDUGuard, and then trains you on how to configure this simple yet powerful feature.

Kevin Wallace, CCIEx2 (R/S and Collaboration) #7945

If you enjoyed this article, you might also want to subscribe to my podcast:

iTunes: http://kwtrain.com/podcast

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