Posts Tagged ‘eigrp’


Continuing my review of titles from Petr’s excellent CCDE reading list for his upcoming LIVE and ONLINE CCDE Bootcamps, here are further notes to keep in mind regarding EIGRP.

About the Protocol

  • The algorithm used for this advanced Distance Vector protocol is the Diffusing Update Algorithm.
  • As we discussed at length in this post, the metric is based upon Bandwidth and Delay values.
  • For updates, EIGRP uses Update and Query packets that are sent to a multicast address.
  • Split horizon and DUAL form the basis of loop prevention for EIGRP.
  • EIGRP is a classless routing protocol that is capable of Variable Length Subnet Masking.
  • Automatic summarization is on by default, but summarization and filtering can be accomplished anywhere inside the network.

Neighbor Adjacencies

EIGRP forms “neighbor relationships” as a key part of its operation. Hello packets are used to help maintain the relationship. A hold time dictates the assumption that a neighbor is no longer accessible and causes the removal of topology information learned from that neighbor. This hold timer value is reset when any packet is received from the neighbor, not just a Hello packet.

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To start my reading from Petr’s excellent CCDE reading list for his upcoming LIVE and ONLINE CCDE Bootcamps, I decided to start with:
EIGRP for IP: Basic Operation and Configuration by Russ White and Alvaro Retana
I was able to grab an Amazon Kindle version for about $9, and EIGRP has always been one of my favorite protocols.
The text dives right in to none other than the composite metric of EIGRP and it brought a smile to my face as I thought about all of the misconceptions I had regarding this topic from early on in my Cisco studies. Let us review some key points regarding this metric and hopefully put some of your own misconceptions to rest.

  • While we are taught since CCNA days that the EIGRP metric consists of 5 possible components – BW, Delay, Load, Reliability, and MTU; we realize when we look at the actual formula for the metric computation, MTU is actually not part of the metric. Why have we been taught this then? Cisco indicates that MTU is used as a tie-breaker in a situation that might require it. To review the actual formula that is used to compute the metric, click here.
  • Notice from the formula that the K (constant values) impact which components of the metric are actually considered. By default K1 is set to 1 and K3 is set to 1 to ensure that Bandwidth and Delay are utilized in the calculation. If you wanted to make Bandwidth twice as significant in the calculation, you could set K1 to 2, as an example. The metric weights command is used for this manipulation. Note that it starts with a TOS parameter that should always be set to 0. Cisco never did fully implement this functionality.
  • The Bandwidth that effects the metric is taken from the bandwidth command used in interface configuration mode. Obviously, if you do not provide this value – the Cisco router will select a default based on the interface type.
  • The Delay value that effects the metric is taken from the delay command used in interface configuration mode. This value depends on the interface hardware type, e.g. it is lower for Ethernet but higher for Serial interfaces. Note how the Delay parameter allows you to influence EIGRP pathing decisions without the manipulation of the Bandwidth value. This is nice since other mechanisms could be relying heavily on the bandwidth setting, e.g. EIGRP bandwidth pacing or absolute QoS reservation values for CBWFQ.
  • The actual metric value for a prefix is derived from the SUM of the delay values in the path, and the LOWEST bandwidth value along the path. This is yet another reason to use more predictive Delay manipulations to change EIGRP path preference.

In the next post on the EIGRP metric, we will examine this at the actual command line, and discuss EIGRP load balancing options. Thanks for reading!

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This publication briefly covers the use of 3rd party next-hops in OSPF, RIP, EIGRP and BGP routing protocols. Common concepts are introduced and protocol-specific implementations are discussed. Basic understanding of the routing protocol function is required before reading this blog post.


Third-party next-hop concept appears only to distance vector protocol, or in the parts of the link-state protocols that exhibit distance-vector behavior. The idea is that a distance-vector update carries explicit next-hop value, which is used by receiving side, as opposed to the “implicit” next-hop calculated as the sending router’s address – the source address in the IP header carrying the routing update. Such “explicit” next-hop is called “third-party” next-hop IP address, allowing for pointing to a different next-hop, other than advertising router. Intitively, this is only possible if the advertising and receiving router are on a shared segment, but the “shared segment” concept could be generalized and abstracted. Every popular distance-vector protocols support third party next-hop – RIPv2, EIGRP, OSPF and BGP all carry explicit next-hop value. Look at the figure below – it illustrates the situation where two different distance-vector protocols are running on the shared segment, but none of them runs on all routers attached to the segment. The protocols “overlap” at a “pivotal” router and redistribution is used to provide inter-protocol route exchange.

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In the first part of this series, we subdivided the processes of EIGRP into four discrete steps, and detailed troubleshooting the first two. This is taken from the 5-Day CCNP bootcamp:

  • Discovery of neighbors
  • Exchange of topology information
  • Best path selection
  • Neighbor and topology table maintenance

Let us now discuss path selection and maintenance troubleshooting.

We should all remember that we can view the topology table of EIGRP with the command show ip eigrp topology. Here we can see the successor routes (these are the best routes that are placed in the routing table) and we can see the second best routes, the feasible successor routes. These feasible successor routes are the key to the lightening fast convergence that EIGRP can offer us. When a speaker loses its successor, it can quickly install a feasible successor route in its place.

We need to remember the important rule of feasible successors. The advertised distance of the proposed feasible successor must be less than the feasible distance of the current successor route. This is actually a loop prevention mechanism.

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CCNA students can typically rattle off the fact that EIGRP uses Bandwidth and Delay in its composite metric calculation by default. In fact, they tend to know this as well as their own last name. But I often notice they might have some pretty big misconceptions about how this metric is really calculated, and how they can manipulate it.

Here are some very important “Core Knowledge” facts that we need to keep in mind about the EIGRP metric: Continue Reading

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Every once in a while I come across a tip that is so exciting I want to share it with the world. I was recently going through one of the many posts I read, and saw the answer to a question that I have been wondering about for many years. Awesome job to Steve Shaw who came up with this. Here is the scenario. Continue Reading

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EIGRP is based on the concept of diffusing computations. When something changes in network topology, the routers that detect a loss of network prefix will send out EIGRP QUERY messages that propagate in circular waves similar to the ripples on water surface. Every queried router will in turn query its neighbors and so on, until all routers that knew about the prefix affected. After this, the expanding circle will start collapsing back with EIGRP REPLY messages. The maximum radius of that circle may be viewed as the query scope. From scalability standpoint, it is very important to know what conditions will limit the average query scope, as this directly impact the network stability. You may compare the “query scope” with the concept of flooding domain in OSPF or ISIS. However, in contrast with the link-state protocols, you are very flexible with chosing the query scope boundaries, which is a powerful feature of EIGRP.

There are four conditions that affect query propagation. Almost all of them are based on the fact that query stops once the queried router cannot find the exact match for the requested subnet in its topology table. After this the router responds back that the network is unknown. Based on this behavior, the following will stop query from propagation
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The problem of unequal-cost load-balancing

Have you ever wondered why among all IGPs only EIGRP supports unequal-cost load balancing (UCLB)? Is there any special reason why only EIGRP supports this feature? Apparently, there is. Let’s start with the basic idea of equal-cost load-balancing (ECLB). This one is simple: if there are multiple paths to the same destination with equal costs, it is reasonable to use them all and share traffic equally among the paths. Alternate paths are guaranteed to be loop-free, as they are “symmetric” with respect to cost to the primary path. If we there are multiple paths of unequal cost, the same idea could not be applied easily. For example, consider the figure below:


Suppose there is a destination behind R2 that R1 routes to. There are two paths to reach R2 from R1: one is directly to R2, and another via R3. The cost of the primary path is 10 and the cost of the secondary path is 120. Intuitively, it would make sense to start sending traffic across both paths, in proportion 12:1 to make the most use of the network. However, if R3 implements the same idea of unequal cost load balancing, we’ve got a problem. The primary path to reach R2 heading from R3 is via R1. Thus, some of the packets that R1 sends to R2 via R3 will be routed back to R1. This is the core problem of UCLB: some secondary paths may result in routing loops, as a node on the path may prefer to route back to the origin.
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DMVPN stands for Dynamic Multipoint VPN and it is an effective solution for dynamic secure overlay networks. In short, DMVPN is combination of the following technologies:

1) Multipoint GRE (mGRE)
2) Next-Hop Resolution Protocol (NHRP)
4) Dynamic Routing Protocol (EIGRP, RIP, OSPF, BGP)
3) Dynamic IPsec encryption
5) Cisco Express Forwarding (CEF)

Assuming that reader has a general understanding of what DMVPN is and a solid understanding of IPsec/CEF, we are going to describe the role and function of each component in details. In this post we are going to illustrate two major phases of DMVPN evolution:

1) Phase 1 – Hub and Spoke (mGRE hub, p2p GRE spokes)
2) Phase 2 – Hub and Spoke with Spoke-to-Spoke tunnels (mGRE everywhere)

As for DMVPN Phase 3 – “Scalable Infrastructure”, a separate post is required to cover the subject. This is due to the significant changes made to NHRP resolution logic (NHRP redirects and shortcuts), which are better being illustrated when a reader has good understanding of first two phases. However, some hints about Phase 3 will be also provided in this post.

Note: Before we start, I would like to thank my friend Alexander Kitaev, for taking time to review the post and providing me with useful feedback.

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UPDATE: For more information on Redistribution see the video series Understanding Route Redistribution – Excerpts from CCIE R&S ATC

Simple Redistribution Step-by-Step

We’re going to take our basic topology from the previous post Understanding Redistribution Part I , and configure to provide full connectivity between all devices with the most simple configuration. Then we are going to tweak some settings and see how they affect redistribution and optimal routing. This is going to be an introductory example to illustrate the redistribution control techniques mentioned previously.

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