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Institute of Technology,
Sligo Dept of Computing
Routing & IGRP
Semester 3, Chapter 5
Table of Contents
Network Layer Basics
Routing & Routed Protocols
IP Routing Protocols
IGRP Operation
Institute of Technology,
Sligo Dept of Computing
Network Layer Basics
Path Determination
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A router’s main responsibility when routing packets
is its path determination function.
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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?
Once the router has determined the path, what function
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
Institute of Technology,
Sligo Dept of Computing
Routed & Routing Protocols
Routed Protocols
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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?
Routing Protocols
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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?
Multiprotocol Routing
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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)
Routed & Routing Review
IP
IPX
Apple
Dist-V
RIP
IGRP
Interior
Link-S
OSPF
Hybrid
EIGRP
Exterior
EGP
BGP
Institute of Technology,
Sligo Dept of Computing
IP Routing Protocols
Primary Goals of Routing Protocols

Optimal Route—pick the best route

Efficiency—minimal use of bandwidth and router
processor resources
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Rapid Convergence—the faster, the better.
Some are quicker at converging than others.
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Flexibility—can handle a wide variety of
situations such as high usage and failed routes
Interior vs. Exterior
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Interior Routing protocols operated within an
Autonomous System. What are some examples?
Exterior Routing protocols operated between an
Autonomous System. What are some examples?
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….?
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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?
Dynamic v. Static
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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…
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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.
Configuring Routing Protocols
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Two steps:
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Under global configuration mode…
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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 sub configuration mode…
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Add the directly connected network numbers
Router(config-router)# network A.B.C.D.
A.B.C.D. is the network address (i.e. 172.16.0.0)
Institute of Technology,
Sligo Dept of Computing
IGRP Operation
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…
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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.
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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
By default, IGRP uses bandwidth and delay at equal
cost.
Configuring IGRP
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Two steps:
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Under global configuration mode…
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Enable IGRP routing
Router(config)# router igrp 100
100 is the Autonomous System number.
In router subconfiguration mode…
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Add the directly connected network numbers
Router(config-router)# network 172.16.0.0