EIGRP Fundamentals | CCNA Practical Studies (Cisco Certification & Training)

Enhanced Interior Gateway Protocol (EIGRP) is a proprietary hybrid routing protocol developed by Cisco Systems. EIGRP uses the same distance vector algorithm and distance information as IGRP. However, as its name implies, EIGRP has been enhanced in convergence properties and operating efficiency over IGRP. Principally, EIGRP has been enhanced to use more advanced features to avoid routing loops and to speed convergence time. In addition, EIGRP transmits the subnet mask for each routing entry, enabling EIGRP to support features such as VLSM and route summarization.

EIGRP Features

EIGRP provides advanced features over its predecessors IGRP and RIP:

Increased network width With IP RIP, the largest possible width of your network is 15 hops. When IP EIGRP is enabled, the largest possible width is 224 hops.
Fast convergence EIGRP uses an algorithm called the Diffusing Update Algorithm (DUAL). This algorithm guarantees loop-free operation at every instant throughout a route computation and allows all routers involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in recomputations. DUAL provides a system for routers to not only calculate the best current route to each subnet, but also to calculate alternative routes that could be used if the current route fails. The alternate route, called the feasible successor route, is guaranteed to be loop-free, so convergence can happen quickly. Because of DUAL, the convergence time of EIGRP rivals that of other existing routing protocols.
Partial updates EIGRP sends incremental updates when the state of a destination changes, instead of sending the entire contents of the routing table. This feature reduces the bandwidth required for EIGRP packets and also reduces CPU processing.
Neighbor-discovery mechanism This is a simple hello mechanism used to learn about neighboring routers. It is protocol-independent .
VLSM and route summarization EIGRP supports variable-length subnet masks and route summarization.
Automatic redistribution Because IGRP and EIGRP share the same metrics, IP IGRP routes can be automatically redistributed into EIGRP, and IP EIGRP routes can be automatically redistributed into IGRP. If desired, you can turn off redistribution. Redistribution is covered in more detail in Chapter 11, "Route Redistribution."

EIGRP Components

EIGRP has four basic components:

Neighbor discovery/recovery Routers use this process to dynamically learn of other routers on their directly attached networks. Routers also must discover when their neighbors become unreachable or inoperative. Neighbor discovery/recovery is achieved with low overhead by periodically sending small hello packets. As long as hello packets are received, a router can determine that a neighbor is alive and functioning. When this status is determined, the neighboring routers can exchange routing information.
Reliable transport protocol This protocol is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. It supports intermixed transmission of multicast and unicast packets. Some EIGRP packets must be transmitted reliably and others need not be. For efficiency, reliability is provided only when necessary. For example, on a multi-access network that has multicast capabilities, such as Ethernet, it is not necessary to send hellos reliably to all neighbors individually. Therefore, EIGRP sends a single multicast hello with an indication in the packet informing the receivers that the packet need not be acknowledged . Other types of packets, such as updates, require acknowledgment, and this is indicated in the packet. The reliable transport has a provision to send multicast packets quickly when there are unacknowledged packets pending. Doing so helps ensure that convergence time remains low in the presence of varying speed links.
DUAL finite state machine This embodies the decision process for all route computations . It tracks all routes advertised by all neighbors. DUAL uses the distance information, known as a metric, to select efficient, loop-free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least-cost path to a destination that is guaranteed not to be part of a routing loop. When there are no feasible successors but there are neighbors advertising the destination, a recomputation must occur. This is called "going active." A router asks all of its neighbors if they have a feasible successor to the destination. If none replies, the neighbors go active and the process repeats. This is the process whereby a new successor is determined. The amount of time that it takes to recompute a route affects the convergence time. Even though the recomputation is not processor- intensive , it is advantageous to avoid recomputation if it is not necessary. When a topology change occurs, DUAL tests for changes to feasible successors. If feasible successors exist, it uses any that it finds, to avoid unnecessary recomputation.

EIGRP Concepts

The sections that follow describe these EIGRP concepts in greater detail:

Neighbor tables
Topology tables
Feasible successors
Route states
Packet formats
Internal vs. external routes
DUAL

Neighbor Tables

Each router keeps state information about adjacent neighbors. When newly discovered neighbors are learned, the address and interface of the neighbor are recorded. This information is stored in the neighbor table. When a neighbor sends a hello packet, it advertises a hold time, which is the amount of time that a router treats a neighbor as reachable and operational. In other words, if a hello packet isn't heard within the hold time, the hold time expires and DUAL is informed of the topology change.

Topology Tables

The topology table contains all destinations advertised by neighboring routers. Associated with each entry are the destination address and a list of neighbors that have advertised the destination. For each neighbor, the advertised metric is recorded. This is the metric that the neighbor stores in its routing table. If the neighbor is advertising this destination, it must be using the route to forward packets.

Also associated with the destination is the metric that the router uses to reach the destination. This is the sum of the best-advertised metric from all neighbors, plus the link cost to the best neighbor. This is the metric that the router uses in the routing table and when advertising to other routers.

Feasible Successors

A destination entry is moved from the topology table to the routing table when there is a feasible successor. All minimum-cost paths to the destination form a set. From this set, the neighbors that have an advertised metric less than the current routing table metric are considered feasible successors. A router views feasible successors as neighbors that are downstream with respect to the destination. These neighbors and the associated metrics are placed in the forwarding table. When a neighbor changes the metric that it has been advertising or a topology change occurs in the network, the set of feasible successors might have to be re-evaluated. However, this is not categorized as a route recomputation.

Route States

A topology table entry for a destination can have one of two states:

Passive A route is considered in passive state when a router is not performing a route recomputation.
Active A route is in active state when a router is undergoing a route recomputation.

If there are always feasible successors, a route never has to go into active state and it avoids a route recomputation. When there are no feasible successors, a route goes into active state and a route recomputation occurs. A route recomputation commences with a router sending a query packet to all neighbors. Neighboring routers either can reply if they have feasible successors for the destination or optionally can return a query indicating that they are performing a route recomputation. While in active state, a router cannot change the next -hop neighbor that it is using to forward packets. When all replies are received for a given query, the destination can transition to passive state and a new successor can be selected. When a link to a neighbor that is the only feasible successor goes down, all routes through that neighbor commence a route recomputation and enter the active state.

Packet Formats

EIGRP uses the following five packet types:

Hello/Acks Hello packets are sent for neighbor discovery/recovery and do not require acknowledgment.
Updates Update packets are used to convey reachability of destinations. When a new neighbor is discovered, update packets are sent so that the neighbor can build up its topology table.
Queries Query packets are sent when a destination has no feasible successors.
Replies Reply packets are sent when a destination has no feasible successors and are sent in response to query packets to instruct the originator not to recompute the route because feasible successors exist.
Requests Request packets are used to get specific information from one or more neighbors.

Internal Versus External Routes

EIGRP has the notion of internal and external routes. Internal routes have been originated within an EIGRP autonomous system (AS). Therefore, a directly attached network that is configured to run EIGRP is considered an internal route and is propagated with this information throughout the EIGRP AS. External routes have been learned by another routing protocol or reside in the routing table as static routes. These routes are tagged individually with the identity of their origination. Internal EIGRP routes are denoted in the routing table with the letter D preceding the route. External EIGRP routes are denoted in the routing table with a "D EX" preceding the route.

DUAL Example

The topology in Figure 10-1 illustrates how the DUAL algorithm converges. The example focuses on destination to Router X only. Each node shows its cost to X in hops. The arrows show the node's successor. For example, Router C uses Router A to reach X, and the cost is 2.

Figure 10-1. Example of DUAL Convergence

graphics/10fig01.gif

If the link between routers A and B fails, Router B sends a query informing its neighbors that it has lost its feasible successor. Router D receives the query and determines whether it has any other feasible successors. If it does not, it must start a route computation and enter the active state. However, in this case, Router C is a feasible successor because its cost (2) is less than Router D's current cost (3) to destination Router X. Router D can switch to Router C as its successor. In this scenario, routers A and C did not participate because they were unaffected by the change.

Now let's cause a route computation to occur. In this scenario, the link between routers A and C fails. Router C determines that it has lost its successor and that it has no other feasible successors. Router D is not considered a feasible successor because its advertised metric (3) is greater then C's current cost (2) to reach destination Router X. Router C must perform a route computation for destination Router X. Router C sends a query to its only neighbor, Router D. Router D replies because its successor has not changed. Router D does not need to perform a route computation. When Router C receives the reply, it knows that all neighbors have processed the news about the failure to destination Router X. At this point, Router C can choose its new feasible successor of Router D, with a cost of 4 to reach destination Router X. Note that routers A and B were unaffected by the topology change, and Router D needed only to reply to Router C.

This completes the introductory text about EIGRP. Next, review the lab objective that you will accomplish in this chapter.

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