Cisco CCNA Exam Tutorial: IGRP And Equal Cost Load Balancing

To pass the CCNA exam, you've got to know the role of the bandwidth command with IGRP and EIGRP and when to use it. In this tutorial, we'll configure IGRP over a frame relay hub-and-spoke network using the following networks:

R1 (the hub), R2, and R3 are running IGRP over the 172.12.123.0 /24 network. This is a T1 line.

R1 and R3 are also connected on a different subnet, 172.12.13.0 /24. The bandwidth of this connection is 512 KBPS.

R2 and R3 are also connected by an Ethernet segment, 172.23.0.0 /16.

We'll configure IGRP on R1, R2, and R3 with the router igrp 1 command. IGRP will run on all interfaces in the 172.12.0.0 and 172.23.0.0 network.

R1#conf t

R1(config)#router igrp 1

R1(config-router)#network 172.12.0.0

The “1” in the router igrp command refers to the Autonomous System (AS). IGRP is a classful routing protocol, so wildcard masks are not used in the network statements.

R2#conf t

R2(config-if)#router igrp 1

R2(config-router)#network 172.12.0.0

R2(config-router)#network 172.23.0.0

R3#conf t

R3(config-if)#router igrp 1

R3(config-router)#network 172.12.0.0

R3(config-router)#network 172.23.0.0


Run show ip route on R1. R1 will see three equal-cost paths to the Ethernet network. IGRP supports load-sharing over up to four equal-cost paths by default, so all three paths appear in the routing table. R1 will also see a route to the loopback address on R2 and two routes to the loopback address on R3. (You can also run show ip route igrp in order to see only the IGRP routes.)

R1#show ip route igrp


I 172.23.0.0/16 [100/8576] via 172.12.123.2, 00:00:02, Serial0

[100/8576] via 172.12.13.3, 00:00:02, Serial1

[100/8576] via 172.12.123.3, 00:00:01, Serial0

Remember that the numbers in the brackets following the network number in the routes are the Administrative Distance and the IGRP metric, in that order.

Note that classful masks are in use. IGRP does not support variable-length subnet masks (VLSM).

There are two serial connections between R1 and R3. IGRP is assuming that both lines are T1 lines, running at 1544 KBPS. The 172.12.13.0 network is participating in equal-cost load sharing because of IGRP’s bandwidth assumption - that all serial interfaces are connected to T1 lines.

To give IGRP a more accurate picture of the network’s bandwidth, configure bandwidth 512 on R1 and R3’s Serial1 interface (the interfaces on the 172.12.13.0 network).

R1#conf t

R1(config)#interface serial1

R1(config-if)#bandwidth 512

R3#conf t

R3(config)#interface serial 1

R3(config-if)#bandwidth 512

IGRP’s assumption that all serial lines run at 1544 KBPS is overridden by the bandwidth 512 command. IGRP now believes this line runs at 512 KBPS.

To see the effect of this command, clear your routing table on R1.

R1#clear ip route *


R1#show ip route igrp

I 172.23.0.0/16 [100/8576] via 172.12.123.3, 00:00:24, Serial0/0

[100/8576] via 172.12.123.2, 00:00:17, Serial0/0

The routing table is cleared with clear ip route *. To see only the routes received in IGRP updates instead of the entire table, run show ip route igrp.

One of the paths to 172.23.0.0 is now gone - the route that went through the 172.12.13.0 network. Now that IGRP sees that link as slower than the others, equal-cost load balancing will not occur over the 172.12.13.0 network.

It’s important to understand that the bandwidth command does not actually change the bandwidth of the connection; it changes IGRP’s assumption of what the bandwidth is.

In the next part of this IGRP load-balancing tutorial, we'll take a look at how to configure unequal-cost load balancing.

Cisco CCNA / CCNP Certification Exam Review: Protocol Basics

To earn your Cisco CCNA certification and pass the BSCI CCNP exam, you have to know your protocol basics like the back of your hand! To help you review these important concepts, here's a quick look at the basics of RIPv1, RIPv2, IGRP, and EIGRP.

RIPv1: Broadcasts updates every 30 seconds to the address 255.255.255.255. RIPv1 is a classful protocol, and it does not recognize VLSM, nor does it carry subnet masking information in its routing updates. Update contains entire RIP routing table. Uses Bellman-Ford algorithm. Allows equal-cost load-balancing by default. Max hop count is 15. Does not support clear-text or MD5 authentication of routing updates. Updates carry 25 routes maximum.

RIPv2: Multicasts updates every 30 seconds to the address 224.0.0.9. RIPv2 is a classless protocol, allowing the use of subnet masks. Update contains entire RIP routing table. Uses Bellman-Ford algorithm. Allows equal-cost load-balancing by default. Max hop count is 15. Supports clear-text and MD5 authentication of routing updates. Updates carry 25 routes maximum.

IGRP: Broadcasts updates every 90 seconds to the address 255.255.255.255. IGRP is a Cisco-proprietary protocol, and is also a classful protocol and does not recognize subnet masking. Update contains entire routing table. Uses Bellman-Ford algorithm. Equal-cost load-balancing on by default; unequal-cost load-sharing can be used with the variance command. Max hop count is 100.

EIGRP: Multicasts full routing table only when an adjacency is first formed. Multicasts updates only when there is a change in the network topology, and then only advertises the change. Multicasts to 224.0.0.10 and allows the use of subnet masks. Uses DUAL routing algorithm. Unequal-cost load-sharing available with the variance command.

By mastering the basics of these protocols, you're laying the foundation for success in the exam room and when working on production networks. Pay attention to the details and the payoff is "CCNA" and "CCNP" behind your name!

Cisco CCNA / CCNP Certification Exam: Attending A Video Boot Camp

When you're studying for the CCNA and CCNP exams, you've got a lot of different choices when it comes to training. One popular choice is choosing one of the many "boot camps" and five-day in-person courses that are out there. I've taught quite a few of these, and while many of them are good, they do have drawbacks.

Of course, one is cost. Many employers are putting the brakes on paying for CCNA and CCNP boot camps, and most candidates can't afford to pay thousands of dollars for such a class. Then you've got travel costs, meals, and having to possibly burn your own vacation time to take the class. Add in time away from your family and boot camps become impractical for many CCNA / CCNP candidates.

Another issue is fatigue. I enjoy teaching week-long classes, but let's face facts - whether you're training for the CCNA or CCNP exams, you're going to get a lot of information thrown at you in just a few days. You're going to be mentally and physically exhausted at the end of the week, and that's when some boot camps actually have you take the exam! You've got to be refreshed and rested when you take the exam to have your best chance of success.

How can you get the benefit of an experienced instructor without paying thousands of dollars? By taking a Video Boot Camp! There are some high-quality computer-based training (CBT) courses out there, and these courses offer quite a few advantages for the CCNA and CCNP candidate. These courses run hundreds instead of thousands of dollars, and you can train on your own schedule. It is important for you to make and keep that schedule, but instead of spending thousands of dollars and having to travel, you can get world-class CCNA and CCNP training in the comfort of your own home.

By combining a high-quality CCNA or CCNP CBT or video boot camp with a strong work ethic, you're on your way to passing the exam and accelerating your career. Now get to work!