Year 2 - Chapter 5/Cisco 3 - Module 5
Routing Protocols: IGRP
By
Carl Marandola
Copyright 2002
Objectives
• Describe the routing functions of the network
layer and how these functions relate to path
determination in a router
• Describe routed and routing protocols
• Describe interior and exterior protocols
• Describe routing protocol characteristics and
configuration
• Describe IGRP features, operation, and
configuration tasks
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Network Layer Basics
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Network Layer Path Determination
Routing Tables
The Network Layer Communication Path
Addressing: The Network and the Host
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The Goals of Routing
Protocols
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The Optimal (Best) Route
Simplicity and Efficiency
Robustness
Rapid Convergence
Flexibility
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Routing Table
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Path Determination
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Network and Host Addressing
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Path Selection and Packet
Switching
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Static Versus Dynamic Routes
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Routing Metrics
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Complex Metrics
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Addresses and Packet
Forwarding
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Routed Versus Routing
Protocol
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Multiprotocol Routing
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Exterior Versus Interior
Protocols
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Three Classes of Routing
Protocols
• Distance Vector
• Link State
• Hybrid
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Issues in Choosing a
Routing Protocol
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Network size and complexity
Network traffic levels
Security needs
Reliability needs
Network delay characteristics
Organizational policies
Organizational acceptance of change
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Distance-Vector Routing
Basics
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Distance-Vector Topology
Changes
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Understanding IGRP
Operation
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Interior, System, and Exterior IGRP Routes
Creating the IGRP Routing Process
Enhancing IGRP Stability
IGRP Metric Information
IGRP Updates
Maximum Hop Count
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IGRP Overview
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IGRP Key Information
• Cisco proprietary protocol
– Requires all Cisco devices.
• Distance-vector routing protocol
• Periodic updates every 90 seconds
– Full route table exchanged.
– Declares a route inaccessible after three missed update
periods (270 seconds).
– Route removed after seven update periods (630 seconds).
• Metric based on bandwidth and delay by default
– Optional: Can include reliability, load, and MTU.
• Large network diameter – 100 hops (routers)
– Optional: Up to 255 possible.
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Interior and Exterior Routes
Interior routes
Exterior route
Internet
ISP
Typically ISPs and clients do not
share routing information or
protocols. Exterior routes are
typically a default route to the
ISP and static route(s) to the AS.
All routers running IGRP or
Enhanced IGRP (EIGRP) and
using the same AS number will
exchange route information.
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An AS is a collection of networks
under common administration sharing
a common routing strategy. Typically
this would be a company, but a large
company could create multiple ASs.
IGRP Configuration
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Configuring IGRP Example
Router(config)#router igrp 100
Router(config-router)#network 219.17.100.0
Router(config-router)#network 199.6.13.0
Router(config-router)#network 201.100.11.0
If you want the bandwidth on a serial link to be interpreted by IGRP as
anything but 1.54 Mbps (T1 – default), you must use the optional bandwidth
command. This command gives IGRP a value to use but does not alter the
actual bandwidth. The following example sets the bandwidth to 256 Kbps:
Router(config)#int s0
Router(config-if)#ip address 219.17.100.1 255.255.255.0
Router(config-if)#bandwidth 256
Router(config-if)#no shutdown
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The show ip route Command
Router#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate
U - per-user static route, o - ODR
Gateway of last resort is not set
C
I
I
C
204.204.7.0/24 is directly connected, Serial1
223.8.151.0/24 [100/8576] via 204.204.7.1, 00:01:06, Serial1
199.6.13.0/24 [100/10476] via 204.204.7.1, 00:01:06, Serial1
210.93.105.0/24 is directly connected, Ethernet0
Administrative Distance
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Metric
Source
Interface
Received On
The show ip protocols
Command
Router#show ip protocols
Routing Protocol is "igrp 100"
Sending updates every 90 seconds, next due in 69 seconds
Invalid after 270 seconds, hold down 280, flushed after 630
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
IGRP maximum hopcount 100
IGRP maximum metric variance 1
Redistributing: igrp 100
Routing for Networks:
219.17.100.0
199.6.13.0
201.100.11.0
Routing Information Sources:
Gateway
Distance
Last Update
Distance: (default is 100)
Administrative Distance
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The debug igrp events
Command
Router#debug ip igrp events
IGRP event debugging is on
Router#
IGRP: sending update to 255.255.255.255 via Ethernet0 (210.93.105.1)
IGRP: Update contains 0 interior, 3 system, and 0 exterior routes.
IGRP: Total routes in update: 3
IGRP: sending update to 255.255.255.255 via Serial1 (204.204.7.2)
IGRP: Update contains 0 interior, 1 system, and 0 exterior routes.
IGRP: Total routes in update: 1
IGRP: received update from invalid source 223.8.151.1 on Ethernet0
IGRP: received update from 204.204.7.1 on Serial1
IGRP: Update contains 0 interior, 2 system, and 0 exterior routes.
IGRP: Total routes in update: 2
Router#undebug all
IGRP event debugging is off
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The debug igrp transactions
Command
Router#debug ip igrp transactions
IGRP protocol debugging is on
Router#
IGRP: received update from invalid source 223.8.151.1 on Ethernet0
IGRP: received update from 204.204.7.1 on Serial1
network 223.8.151.0, metric 8576 (neighbor 1100)
network 199.6.13.0, metric 10476 (neighbor 8476)
IGRP: sending update to 255.255.255.255 via Ethernet0 (210.93.105.1)
network 204.204.7.0, metric=8476
network 223.8.151.0, metric=8576
network 199.6.13.0, metric=10476
IGRP: sending update to 255.255.255.255 via Serial1 (204.204.7.2)
network 210.93.105.0, metric=1100
Router#undebug all
IGRP event debugging is off
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IGRP Solutions for Routing Loops
IGRP features to enhance stability include :
– Hold-down timers
• If Router 1 tells Router 2 that Network A is
unavailable and Router 2 sets a hold-down
timer, it will not accept information about
Network A from other routers until the timer
expires.
– Split horizon
• Router 2 will not send routes to Router 1 that
it learned about from Router 1 (such as
Network A).
– Poison reverse updates
• Typically used with hold-down timers.
• If Network A goes down and Router 1 drops it
from its routing updates, it will take 3 cycles
(270 seconds) for Router 2 to suspect a
problem and 7 cycles (630 seconds) to drop
the route.
• Alternative: Router 1 sends Router 2 an
update on Network A with the hop count set
to 101 – infinity (unreachable) .
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