CCNP Routing Study Guide- P9 doc

Chapter 6IGRP and EIGRP THE CCNP ROUTING EXAM TOPICS COVERED IN THIS CHAPTER ARE AS FOLLOWS: Describe IGRP features and operation Configure IGRP Verify IGRP operation Describe Enhan

Chapter 6

IGRP and EIGRP

THE CCNP ROUTING EXAM TOPICS COVERED

IN THIS CHAPTER ARE AS FOLLOWS:

Describe IGRP features and operation

Configure IGRP

Verify IGRP operation

Describe Enhanced IGRP features and operation

Explain how metrics are used with EIGRP

Explain how DUAL is used with EIGRP

Explain the features supported by EIGRP

Learn how EIGRP discovers, decides, and maintains routes

Explain EIGRP process identifiers

Explain EIGRP troubleshooting commands

Configure EIGRP and verify its operation

Verify route redistribution

So far in this book, we have taken an in-depth look at the ing protocol OSPF and shown how a routing protocol is used to find routes through the network We also learned how routing protocols are used to exchange IP address information between routers in an enterprise network

rout-IP addressing schemes establish a hierarchy that makes path information both distinct and efficient A router receives this routing information via a given interface It then advertises the information it knows out the other physical interfaces This routing process occurs at Layer 3 of the OSI model

In this chapter, in order to decide on the best routing protocol or protocols to use, we’ll take a look at both the Interior Gateway Routing Protocol (IGRP) and its big brother, the Enhanced Interior Gateway Routing Pro-tocol (EIGRP)

Unlike OSPF, IGRP and EIGRP are proprietary Cisco protocols and run

on Cisco routers and internal route processors found in the Cisco tion and Core layer switches (I need to note here that Cisco has licensed IGRP to be used on other vendors’ equipment, such as Compaq.) Each of these routing protocols also has its own identifiable functions, so we’ll dis-cuss each routing protocol’s features and differences Once you understand how these protocols differ from OSPF and how they calculate routes, you will learn how to configure these protocols and fine-tune them with config-uration changes to make each perform at peak efficiency

Distribu-Scalability Features of Routing Protocols 205

Scalability Features of Routing Protocols

Several times in this book, as we look at the different routing cols—OSPF, IGRP, EIGRP, and BGP—we will refer back to distance-vector and link-state routing protocol differences It is important to identify how these protocols differ from one another

proto-As networks grow and administrators implement or use Cisco-powered networks, OSPF might not be the most efficient or recommended protocol to use OSPF does have some advantages of IGRP, EIGRP, and BGP, including:

 It is versatile

 It uses a very scalable routing algorithm

 It allows the use of a routing protocol that is compatible with Cisco routers

non-BGP will be discussed in Chapters 7 through 9

Cisco provides two other proprietary solutions that allow better scaling and convergence, which can be very critical issues These are the Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP) Network growth imposes a great number of changes on the network environment and takes into consideration the following factors:

 The number of hops between end systems

 The number of routes in the routing table

 The different ways a route was learned

 Route convergence IGRP and EIGRP can be used to maintain a very stable routing environment, which is absolutely crucial in larger networks

As the effects of network growth start to manifest themselves, whether or not your network’s routers can meet the challenges faced in a larger scaled network is completely up to the routing protocol the routers are running If you use a protocol that’s limited by the number of hops it can traverse, the

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number of routes it can store in its table, or even the inability to cate with other protocols, then you have a protocol that will likely hinder the growth of your network

communi-All the issues we’ve brought up so far are general scalability ations Before we look at IGRP and EIGRP, let’s take another look at the dif-ferences between link-state routing protocols and distance-vector protocols and the scalability issues of each

consider-Link-state routing and distance-vector protocols are discussed in detail in Chapter 2, and are discussed in Chapter 7 as they relate to BGP

Distance-Vector Protocol Scalability Issues

In small networks—meaning those with fewer than 100 routers and an ronment that’s much more forgiving of routing updates and calculations—distance-vector protocols perform fairly well However, you’ll run into sev-eral problems when attempting to scale a distance-vector protocol to a larger network—convergence time, router overhead (CPU utilization), and band-width utilization all become factors that hinder scalability

envi-A network’s convergence time is determined by the ability of the protocol

to propagate changes within the network topology Distance-vector protocols don’t use formal neighbor relationships between routers A router using distance-vector algorithms becomes aware of a topology change in two ways:

 When a router fails to receive a routing update from a directly nected router

con- When a router receives an update from a neighbor notifying it of a topology change somewhere in the network

Routing updates are sent out on a default or specified time interval So when a topology change occurs, it could take up to 90 seconds before a neighboring router realizes what’s happened When the router finally recog-nizes the change, it recalculates its routing table and sends the whole thing out to all its neighbors

Not only does this cause significant network convergence delay, it also devours bandwidth—just think about 100 routers all sending out their entire routing table and imagine the impact on your bandwidth It’s not exactly a

Scalability Features of Routing Protocols 207

sweet scenario, and the larger the network, the worse it gets, because a greater percentage of bandwidth is needed for routing updates

As the size of the routing table increases, so does CPU utilization, because

it takes more processing power to calculate the effects of topology changes and then converge using the new information Also, as more routes populate

a routing table, it becomes increasingly complex to determine the best path and next hop for a given destination The following list summarizes the scal-ability limitations inherent in distance-vector algorithms:

 Network convergence delay

 Increased CPU utilization

 Increased bandwidth utilization

Scalability Limitations of Link-State Routing Protocols

Link-state routing protocols assuage the scalability issues faced by vector protocols because the algorithm uses a different procedure for route calculation and advertisement This enables them to scale along with the growth of the network

distance-Addressing distance-vector protocols’ problem with network gence, link-state routing protocols maintain a formal neighbor relationship with directly connected routers that allows for faster route convergence They establish peering by exchanging Hello packets during a session, which cements the neighbor relationship between two directly connected routers This relationship expedites network convergence because neighbors are immediately notified of topology changes Hello packets are sent at short intervals (typically every 10 seconds), and if an interface fails to receive Hello packets from a neighbor within a predetermined hold time, the neighbor is considered down, and the router will then flood the update out all physical interfaces This occurs before the new routing table is calculated, so it saves time Neighbors receive the update, copy it, flood it out their interfaces, and

conver-then calculate the new routing table The procedure is followed until the topology change has been propagated throughout the network

It’s noteworthy that the router sends an update concerning only the new

information—not the entire routing table So the update is a lot smaller, which saves both bandwidth and CPU utilization Plus, if there aren’t any network changes, updates are sent out only at specified, or default, intervals,

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which differ among specific routing protocols and can range from 30 utes to two hours

min-These are key differences that permit link-state routing protocols to tion well in large networks—they really have no limitations when it comes to scaling, other than the fact that they’re a bit more complex to configure than distance-vector protocols

func-Interior Gateway Routing Protocol

Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary ing protocol that uses a distance-vector algorithm It uses this algorithm because it uses a vector (a one-dimensional array) of information to calculate the best path This vector consists of four elements:

rout- Bandwidth

 Delay

 Load

 ReliabilityWe’ll describe each element in detail shortly

Maximum transfer unit (MTU) information is included in the final route mation, but it’s used as part of the vector of metrics.

infor-IGRP is intended to replace RIP and create a stable, quickly converging protocol that will scale with increased network growth As we mentioned, it’s preferable to implement a link-state routing protocol in large networks because of the overhead and delay that results from using a distance-vector protocol

In the next few sections, we will quickly take you through the features of IGRP and show how to implement this routing protocol in your network

We will also cover the types of metrics, unequal-cost load balancing, and the limitations of redistribution

Interior Gateway Routing Protocol 209

IGRP Features and Operation

IGRP has several features included in the algorithm—these features and a brief description can be found below in Table 6.1 Most of these features were added to make IGRP more stable, and a few were created to deal with routing updates and make network convergence happen faster

IGRP is a classful protocol, which means it doesn’t include any subnet information about the network with route information

Classful protocols are discussed in Chapter 2.

IGRP recognizes three types of routes:

Interior Networks directly connected to a router interface

T A B L E 6 1 IGRP Features

Configurable metrics The user can configure metrics involved

in the algorithm responsible for calculating route information.

Flash update Updates are sent out prior to the default

time setting This occurs when the metrics for a route change.

Poison reverse updates Implemented to prevent routing loops,

these updates place a route in down Hold-down means that the router won’t accept any new route information

hold-on a given route for a certain period

of time.

Unequal-cost load balancing Allows packets to be shared or

distributed across multiple paths.

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System Routes advertised by other IGRP neighbors within the same autonomous system (AS) The AS number (ASN) identifies the IGRP ses-sion, because it’s possible for a router to have multiple IGRP sessions

Exterior Routes learned via IGRP from a different ASN, which provide information used by the router to set the gateway of last resort The gate-way of last resort is the path a packet will take if a specific route isn’t found on the router

When we talked about the scalability of distance-vector protocols, we told you that they don’t establish a formal neighbor relationship with directly connected routers and that routing updates are sent at designated intervals IGRP’s interval is 90 seconds, which means that every 90 seconds IGRP will broadcast its entire routing table to all directly connected IGRP neighbors

 Delay is the sum of all the delays of the links along the paths

 Delay = [Delay in 10s of microseconds] × 256

 BW is the lowest bandwidth of the links along the paths

 BW = [10000000 / (bandwidth in Kbps)] × 256

 By default, metric = bandwidth + delay

The formula above is used for the non-default setting, when K5 does not equal

0 If K5 equals the default value of 0, then this formula is used: metric = K1 × bandwidth + (K2 × bandwidth) / (256 − Load) + K3 × Delay].

If necessary, you can adjust metrics within the router configuration face Metrics are tuned to change the manner in which routes are calculated

inter-Interior Gateway Routing Protocol 211

After you enable IGRP on a router, metric weights can be changed using the following command:

metric weights tos K1 K2 K3 K4 K5

Table 6.2 shows the relationship between the constant and the metric it affects

Each constant is used to assign a weight to a specific variable This means that when the metric is calculated, the algorithm will assign a greater impor-tance to the specified metric By assigning a weight, you are able to specify what is most important If bandwidth is of greatest concern to a network

administrator, then a greater weight would be assigned to K1 If delay is unacceptable, then the K2 constant should be assigned a greater weight The

tos variable is the type of service

As well as tuning the actual metric weights, you can do other tunings All routing protocols have an administrative distance associated with the proto-col type If multiple protocols are running on one router, the administrative distance value helps the router decide which path is best The protocol with the lowest administrative distance will be chosen IGRP has a default admin-istrative distance of 100 The tuning of this value is accomplished with the distance command, like this:

distance 1–255Valid values for the administrative distance range from 1 to 255 Again, the lower the value, the better

T A B L E 6 2 Metric Association of K Values

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When redistributing static routes or other protocol types within IGRP, metrics may be set for these routes as well by using the default-metric command:

default-metric bandwidth delay reliability load MTU

The words in italics in the command above are just placeholders for variables and should be replaced with numbers.

Bandwidth and delay have a range of values from 0 to 4,294,967,295 (in

Kbps) and 0 to 4,294,967,295 (in 10-microsecond units), respectively

Reli-ability ranges from 0 to 255, with 255 being the most reliable Load ranges

from 0 to 255; however, a value of 255 means that the link is completely

loaded Finally, the value of MTU has the same range as the bandwidth

vari-able: 0 to 4,294,967,295

When a router receives multiple routes for a specific network, one of the routes must be chosen as the best route from all of the advertisements The router still knows that it is possible to get to a given network over multiple interfaces, yet all data default to the best route

IGRP provides the ability of unequal-cost load balancing The variance command is used to assign a weight to each feasible successor A feasible suc-cessor is a predetermined route to use should the most optimal path be lost The feasible successor can also be used as long as the secondary route con-forms to the following three criteria, and an unequal-cost load balancing ses-sion may be established:

 A limit of four feasible successors may be used for load balancing Four is the default; the maximum number of feasible successors is six for IOS version 11.0 and later

 The feasible successor’s metric must fall within the specified variance

of the local metric

 The local metric must be greater than the metric for the next-hop router

Enhanced Interior Gateway Routing Protocol 213

A lower metric signifies a better route.

Redistribution Limitations

As an enterprise network grows, there is a possibility that more than one protocol will run on the router An example is when a company acquires another company and needs to merge the two existing networks The prob-lem surfaces when the routes of the purchasing company need to be adver-tised to the newly acquired company IGRP solves the problem with route redistribution

When multiple protocols run on a router, you can configure IGRP to redistribute routes from specified protocols Since different protocols calcu-late metrics distinctly, adjustments must be made when redistributing pro-tocols These adjustments cause some limitations in how the redistribution works The adjustments are made by using the default-metric command,

as shown previously

IGRP may also be redistributed to other routing protocols such as RIP, other IGRP sessions, EIGRP, and OSPF Metrics are also configured using the default-metric command

Enhanced Interior Gateway Routing Protocol

Enhanced Interior Gateway Routing Protocol (EIGRP) is better than its little brother, IGRP EIGRP allows for equal-cost load balancing, incre-mental routing updates, and formal neighbor relationships, which overcome the limitations of IGRP The enhanced version uses the same distance-vector information as IGRP, yet with a different algorithm EIGRP uses DUAL (Diffusing Update Algorithm) for metric calculation, which permits rapid convergence This algorithm allows for the following:

 Backup route determination if one is available

 Support of Variable-Length Subnet Masks (VLSM)

 Dynamic route recoveries

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 Querying neighbors for unknown alternate routes

 Sending out queries for an alternate route if no route can be foundEIGRP fixes many of the problems associated with IGRP, such as the propagation of the entire routing table, which is sent when changes occur in the network topology One unique characteristic of EIGRP is that it is both

a link-state routing and a distance-vector protocol How can this be? Let’s look at how this protocol combines the best from both routing protocol types

Along with rapid convergence discussed above, EIGRP reduces width usage It does this by not making scheduled updates but sending updates only when topology changes occur When EIGRP does send an update, the update contains information only on the change in the topology, which requires a path or metric change Another plus is the fact that only the routers that need to know about the change receive the update

band-One of the best features is that the routing protocol supports all of the major Layer 3 routed protocols using protocol-dependent modules (PDMs), those being IP, IPX, and AppleTalk At the same time, EIGRP can maintain

a completely loop-free routing topology and very predictable behavior, even when using all three routed protocols over multiple redundant links.With all these features, EIGRP must be hard to configure, right? Guess again Cisco has made this part easy as well and allows you to implement load balancing over equal-cost links So why would you use anything else? Well, I guess you might if all your routers weren’t Cisco routers Remember, EIGRP is proprietary and only runs over Cisco routers and internal route processors

Now that we have mentioned all this, we’ve sold you on EIGRP, right? Well, if we stopped right here, you would miss out on many other important details of the route-tagging process, neighbor relationships, route calcula-tion, and the metrics used by EIGRP, which will be discussed in the next few sections Following that discussion, we will look at how to configure EIGRP, tune EIGRP, load balance, redistribute routes, and troubleshoot

Enhanced Interior Gateway Routing Protocol 215

routing information, which includes the routes learned and the ment of topology changes

advertise-Route redistribution, which will be covered in its own section later in this chapter, allows routes learned by one AS EIGRP session to be shared with another session When route distribution occurs, the routes are tagged as being learned from an external EIGRP session Each type of route is assigned its own administrative distance value

Neighbor Relationships

Using Hello messages, EIGRP sessions establish and maintain neighbor tionships with neighboring routers This is a quality of a link-state routing protocol EIGRP uses the Hello protocol just like OSPF does, as discussed in Chapter 5, to establish and maintain the peering relationships with directly connected routers The Hello packets sent between EIGRP neighboring rout-ers determine the state of the connection between them Once the neighbor relationship is established using the Hello protocol, the routers then exchange route information

rela-Each EIGRP session running on a router establishes a neighbor table in which each router stores information on all the routers known to be directly connected neighbors The neighboring routers’ IP address, hold time inter-val, smooth round-trip timer (SRTT), and queue information are all kept in the table, which is used to help determine when there are topology changes that need to be propagated to the neighboring routers

The only time EIGRP advertises its entire routing table is when two bors initiate communication When this happens, both neighbors advertise their entire routing tables to one another After each has learned its neigh-bor’s directly connected or known routes, only changes to the routing table are propagated

neigh-When Hello messages are sent out each of the routers’ interfaces, replies

to the Hello packets are sent with the neighboring router’s topology table (which is not the routing table) and include each route’s metric information with the exception of any routes that were already advertised by the router receiving the reply As soon as the reply is received, the receiving router sends out what is called an ACK (acknowledgement) packet to acknowledge receipt, and the routing table is updated if any new information is received from the neighboring router Once the topology table has been updated, the originating router will then advertise its entire table to any new neighbors

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that come online Then when the originating router receives information from its neighbors, the route calculation process begins Let’s now take a look at how EIGRP uses metrics to calculate the best routes through the network

Route Calculation

EIGRP uses multicasts instead of broadcasts Therefore, only identified tions are affected by routing updates or queries Where IGRP updates use a 24-bit format, EIGRP uses a 32-bit format for granularity Only changes in the network topology are advertised instead of the entire topology table EIGRP is called an advanced distance-vector protocol although it con-tains properties of both distance-vector and link-state routing protocols when calculating routes DUAL is much faster and calculates new routes only when updates or Hello messages cause a change in the routing table And then recalculation occurs only when the changes directly affect the routes contained in the routing table

sta-This last statement may be confusing If a change occurs to a network that

is directly connected to a router, all of the relevant information is used to culate a new metric and route entry for it If a link between two EIGRP peers becomes congested, both routers would have to calculate a new route metric, then advertise the change to any other directly connected routers

cal-Now that we understand the difference between a route update and a route calculation, we can summarize the steps that a router takes to calcu-late, learn, and propagate route update information

Redundant Link Calculation

The topology database stores all known routes to a destination and the rics used to calculate the least-cost path Once the best routes have been cal-culated, they are moved to the routing table The topology table can store up

met-to six routes met-to a destination network, meaning that EIGRP can calculate the best path for up to six redundant paths Using the known metrics to the des-tination, the router must make a decision as to which path to make its pri-mary path and which path to use as a standby or secondary path to a destination network Once the decision is made, the primary route will be

added to the routing table as the active route, or successor, and the standby will be listed as a passive route, or the feasible successor, to the destination.

The path-cost calculation decisions are made from information contained

in the routing table using the bandwidth and delay from both the local and

Enhanced Interior Gateway Routing Protocol 217

adjacent routers Using this information, a composite metric is calculated The local router adds its cost to the cost advertised by the adjacent router The total cost is the metric Figure 6.1 shows how cost is used to select the best route (successor) and the backup route (feasible successor)

F I G U R E 6 1 The best-route selection process

Using RouterA as a starting point, we see that there are three different routes to Host Y Each link has been assigned a cost Numbers in bold rep-

resent advertised distances, and numbers in italics represent feasible

dis-tances Advertised distances are costs that routers advertise to neighbors.

In this example, RouterD and the WAN all have advertised costs that they send to RouterA In turn, RouterA has a feasible distance for every router to which it is connected The feasible distance is the cost assigned to the link that connects adjacent routers

The feasible and advertised costs are added together to provide a total cost to reach a specific network Let’s calculate the lowest cost for Host X to get to Host Y We will use the path from Host X to RouterA to RouterB to Router C and finally to Host Y for our first path calculation To calculate the

WAN CO

Host X 172.7.8.0/24

Cost 30 172.3.4.4/30

172.1.2.4/30 172.6.7.4/30

172.5.6.4/30 172.10.10.0/24

172.11.12.4/30 Cost 35

Cost 35 Cost 20