Enhanced IGRP (EIGRP)

Enhanced Interior Gateway Routing Protocol (EIGRP)

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco protocol that runs on Cisco routers and on some Cisco switches

EIGRP has a number of features that make it especially useful within large, complex networks. A real standout among these is its support of VLSM, which is crucial to its ultraefficient scalability.

Cisco refers to EIGRP as a distance-vector routing protocol but also as an advanced distance-vector or even a hybrid routing protocol.

Features

Neighbor Discovery

Understand that the AS number is irrelevant—that is, as long as all routers use the same number! You can use any number from 1 to 65,535.

Syntax

By using the command network 10.0.0.0, we can effectively advertise to all subnets within that classful network; however, take a look at this configuration now:

Commands

Controlling EIGRP Traffic

EIGRP Tables

The neighbor and topology tables are stored in RAM and maintained through the use of Hello and update packets. While the routing table is also stored in RAM, the information stored in the routing table is gathered only from the topology table.

EIGRP Terms

Successor A successor route—again, think “successful”—is the best route to a remote network. A successor route is the lowest cost to a destination and stored in the topology table along with everything else. However, this particular best route is copied and placed in the routing table so IP can use it to get to the remote network. The successor route is backed up by a feasible successor route, which is also stored in the topology table, if there’s one available. The routing table contains only successor routes; the topology table contains successor and feasible successor routes.

Feasible successor (FS) So a feasible successor is basically an entry in the topology table that represents a path that’s inferior to the successor route(s). An FS is defined as a path whose advertised distance is less than the feasible distance of the current successor and considered a backup route. EIGRP will keep up to 32 feasible successors in the topology table in 15.0 code but only up to 16 in previous IOS versions, which is still a lot! Only the path with the best metric—the successor—is copied and placed in the routing table. The show ip eigrp topology command will display all the EIGRP feasible successor routes known to the router.

A feasible successor is a backup route and is stored in the topology table. A successor route is stored in the topology table and is copied and placed in the routing table.

Reliable Transport Protocol (RTP)

EIGRP depends on a proprietary protocol, called Reliable Transport Protocol (RTP), to manage the communication of messages between EIGRP-speaking routers. As the name suggests, reliability is a key concern of this protocol, so Cisco designed this mechanism, which leverages multicasts and unicasts, to ensure that updates are delivered quickly and that data reception is tracked accurately.

When EIGRP sends multicast traffic, it uses the Class D address 224.0.0.10, and each EIGRP router knows who its neighbors are. For each multicast it sends out, a list is built and maintained that includes all the neighbors who have replied. If a router doesn’t get a reply from a neighbor via the multicast, EIGRP will then try using unicasts to resend the same data. If there’s no reply from a neighbor after 16 unicast attempts, that neighbor will then be declared dead. This process is often referred to as reliable multicast.

Routers keep track of the information they send by assigning a sequence number to each packet that enables them to identify old, redundant information and data that’s out of sequence. You’ll get to actually see this information in the neighbor table coming up when we get into configuring EIGRP.

Remember, EIGRP is all about topology changes and updates, making it the quiet, performance-optimizing protocol it is. Its ability to synchronize routing databases at startup time, while maintaining the consistency of databases over time, is achieved quietly by communicating only necessary changes. The downside here is that you can end up with a corrupted routing database if any packets have been permanently lost or if packets have been mishandled out of order!

Diffusing Update Algorithm (DUAL)

I mentioned that EIGRP uses Diffusing Update Algorithm (DUAL) for selecting and maintaining the best path to each remote network. DUAL allows EIGRP to carry out these vital tasks:

Quite an impressive list, but what really makes DUAL so great is that it enables EIGRP to converge amazingly fast! The key to the speed is twofold: First, EIGRP routers maintain a copy of all of their neighbors’ routes to refer to for calculating their own cost to each remote network. So if the best path goes down, all it often takes to find another one is a quick scan of the topology table looking for a feasible successor. Second, if that quick table survey doesn’t work out, EIGRP routers immediately ask their neighbors for help finding the best path. It’s exactly this, ahem, DUAL strategy of reliance upon, and the leveraging of, other routers’ information that accounts for the algorithm’s “diffusing” character. Unlike other routing protocols where the change is propagated through the entire network, EIGRP bounded updates are propagated only as far as needed.

As you already know, the Hello protocol ensures the rapid detection of new or dead neighbors, and RTP provides a reliable method of conveying and sequencing messages. Based upon this solid foundation, DUAL can then select and maintain information about the best paths. Let’s check further into the process of route discovery and maintenance next.

VLSM Support and Summarization

Being one of the more sophisticated classless routing protocols, EIGRP supports using variable length subnet masks. This is good because it allows us to conserve address space by using subnet masks that map to specific host requirements in a much better way. Being able to use 30-bit subnet masks for the point-to-point networks that I configured in our internetwork is a great example. Plus, because the subnet mask is propagated with every route update, EIGRP also supports the use of discontiguous subnets, giving us greater administrative flexibility when designing a network IP address scheme. Another versatile feature is that EIGRP allows us to use and place route summaries at strategically optimal locations throughout the EIGRP network to reduce the size of the routing table.

Keep in mind that EIGRP automatically summarizes networks at their classful boundaries and supports the manual creation of summaries at any and all EIGRP routers. This is usually a good thing, but by checking out the routing table in the Corp router, you can see the possible complications that auto-summarization can cause:

By default, EIGRP can provide equal-cost load balancing across up to 4 links. RIP and OSPF do this too. But you can have EIGRP actually load-balance across up to 32 links with 15.0 code (equal or unequal) by using the following command:

EIGRP has a default maximum hop count of 100 for route update packets, but it can be set up to 255. Chances are you wouldn’t want to ever change this, but if you did, here is how you would do it:

As you can see from this router output, EIGRP can be set to a maximum of 255 hops. Even though it doesn’t use hop count in the path metric calculation, it still uses the maximum hop count to limit the scope of the AS.

EIGRP Metrics

Unlike many other protocols that use a single element to compare routes and select the best possible path, EIGRP uses a combination of these four factors:

If necessary, you can adjust the constant K values on a per-interface basis, but I would recommend that you only do this under the direction of the Cisco Technical Assistance Center (TAC). Metrics are tuned to change the manner in which routes are calculated. The K values can be seen with a show ip protocols output:

EIGRP Packets

Update An Update packet contains route information. When these are sent in response to metric or topology changes, they use reliable multicasts. In the event that only one router needs an update, like when a new neighbor is discovered, it’s sent via unicasts. Keep in mind that the unicast method still requires an acknowledgment, so updates are always reliable regardless of their underlying delivery mechanism.