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Routing & IGRP
Semester 3, Chapter 5
Table of Contents
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Network Layer Basics
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Routing & Routed Protocols
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IP Routing Protocols
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IGRP Operation
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Network Layer Basics
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Path Determination
 A router’s main responsibility when
routing packets is its path determination
function.
 Path determination is the process of making a forwarding
decision? (At what layer?)
 What do routers use to make a forwarding decision? What
metric counts the number of routers the packet goes
through?
 What are the two parts of a routed protocol?
 Which part do routers use to determine the path?
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 Once the router has determined the path, what function
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allows it to forward it out the correct interface?
Router’s Steps
 Router operation has four basic steps:
1. Open the frame received on the interface and read the MAC
address.
2. If the MAC resides on the same subnet, discard the frame. If the
frame is an ARP request for a destination on a different subnet,
send a RARP. All other broadcasts are discarded. If the MAC
specifies a host on another subnet, open the packet.
3. Read the IP address and look it up in the routing table. Use the
routing protocol(s) configured to determine the best path.
4. Re-encapsulate the packet in the correct frame format with the
next-hop’s data-link layer address and switch the frame out that
interface
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Routed & Routing
Protocols
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Routed Protocols
Routed protocols are
moved across the
network by routing
protocols.
 Each routed protocol has
its own layer 3 packet
header format with its
own unique hierarchical
addressing scheme.
 What are some examples
of routed protocols?

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Routing Protocols
Routing protocols are
used by routers to
communicate with each
other about various paths
to the destination.
 Each routing protocol has
its own set of benefits and
drawbacks.
 What are some examples
of routing protocols?

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Multiprotocol Routing
Routers are capable of using multiple routed and routing
protocols at the same time.
 Through the use of subinterfaces, a router can even use
different routed protocols on the same interface. (See Graphic)

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Routed & Routing Review
IP
IPX
Apple
Dist-V
Interior
Link-S
Hybrid
Exterior
EGP
BGP
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RIP
IGRP
OSPF
EIGRP
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IP Routing Protocols
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Primary Goals of Routing Protocols
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Optimal Route—pick the best route
Efficiency—minimal use of bandwidth and router processor
resources
Rapid Convergence—the faster, the better. Some are quicker
at converging than others.
Flexibility—can handle a wide variety of situations such as
high usage and failed routes
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Interior vs. Exterior
Interior Routing protocols operated within an Autonomous
System. What are some examples?
 Exterior Routing protocols operated between an Autonomous
System. What are some examples?

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Interior Routing Protocols

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Distance-Vector—passes periodic copies of its routing tables
to its neighbor routers. How often for RIP? IGRP?
Link-State—broadcasts or multicasts topology changes when
they occur. Also called SPF routing. SPF stands for….?
Hybrid—a distance-vector routing protocol that does not send
periodic copies of its routing table. Instead, it sends topology
change updates. What protocol is a hybrid?
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Dynamic v. Static
Static routing is not a protocol. The net. admin. statically
configures a route using the ip route command.
 Dynamic routing refers to the use of a routing protocol to
determine the path. In dynamic routing…

Routes are dynamically adjusted as the topology changes.
Accurate routing tables and timely updates are crucial to avoid
routing loops and unacceptable delay.
A lack of convergence (all routers with the same information) is
the main reason for inaccurate routing tables and routing loops.

Routing loops are solved through hold down timers, split
horizons, and poison reverse routes. Describe each of
these solutions.
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Configuring Routing Protocols
 Two steps:
 Under global configuration mode…
Enable the routing protocol
Router(config)# router protocol [processid]
[process-id]is the Autonomous System number for IGRP
and EIGRP. For OSPF, it has a different meaning.

In router subconfiguration mode…
Add the directly connected network numbers
Router(config-router)# network A.B.C.D. Home
A.B.C.D. is the network address (i.e. 172.16.0.0)
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IGRP Operation
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IGRP Metrics
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IGRP is a Cisco proprietary routing protocol with an AD of 100. What’s
meant by proprietary?
IGRP provides a wide range of configurable metrics. Net. Admin. can
set values for…
 Bandwidth--choose the fastest route. If two equal paths, then load balance
 Delay--chose the route with the least amount of cumulative interface delay along
the path
 The following are not normally used because they cause more frequent routing
table calculations.
 Load--similar to delay, based upon total bits per second currently being transmitted
(throughput)
 Reliability--based on keepalives, chose the link that stays up the most
 MTU--choose the link with the largest MTU
 Hop Count--can be as high as 255
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
By default, IGRP uses bandwidth and delay at equal cost.
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Configuring IGRP
 Two steps:
 Under global configuration mode…
Enable IGRP routing
Router(config)# router igrp 100
100 is the Autonomous System number.

In router subconfiguration mode…
Add the directly connected network numbers
Router(config-router)# network 172.16.0.0
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