represents bandwidth and the metric K3 represents
delay. By default the values of the metrics K1 and K3 are set
to 1, while K2, K4 and K5 are set to 0.This composite metric is
more accurate than the hop count metric that RIP uses when
choosing a path to a destination. The path that has the
smallest metric value is the best route. The metrics that IGRP
uses are: - Bandwidth – The lowest bandwidth
value in the path
- Delay – The cumulative
interface delay along the path
- Reliability –
The reliability on the link towards the destination as
determined by the exchange of keepalives
- Load –
The load on a link towards the destination based on bits per
second
- MTU – The Maximum Transmission Unit
value of the path.
IGRP uses a composite metric.
This metric is calculated as a function of bandwidth, delay,
load, and reliability. By default, only bandwidth and delay are
considered. The other parameters are considered only if enabled
via configuration. Delay and bandwidth are not measured values,
but are set via the delay and bandwidth interface commands. The
show ip route command in the example shows the IGRP
metric values in brackets. A link with a higher bandwidth will
have a lower metric, and a route with a lower cumulative delay
will have a lower metric. Web Links Configuring IGRP
http://www.cisco.com/en/US/products/sw/ iosswrel/ps1831/
products_configuration_guide_ chapter09186a00800d97f6.html
Content 7.3 IGRP 7.3.3 IGRP
routes IGRP advertises three types of routes:
Interior
Interior routes are routes between subnets
of a network attached to a router interface. If the network
attached to a router is not subnetted, IGRP does not advertise
interior routes. System
System routes are routes to
networks within an autonomous system. The Cisco IOS software
derives system routes from directly connected network
interfaces and system route information provided by other
IGRP-speaking routers or access servers. System routes do not
include subnet information. Exterior
Exterior routes
are routes to networks outside the autonomous system that are
considered when identifying a gateway of last resort. The Cisco
IOS software chooses a gateway of last resort from the list of
exterior routes that IGRP provides. The software uses the
gateway (router) of last resort if a better route is not found
and the destination is not a connected network. If the
autonomous system has more than one connection to an external
network, different routers can choose different exterior
routers as the gateway of last resort. Interactive Media
Activity Checkbox: IGRP Routes After completing this
activity, the student will be able to understand IGRP routes.
Web Links Interior Gateway Protocol - IGRP
http://www.firewall.cx/ index.php?c=igrp
Content
7.3 IGRP 7.3.4 IGRP stability
features IGRP has a number of features that are designed to
enhance its stability, such as: - Holddowns
- Split horizons
- Poison reverse updates
Holddowns
Holddowns are used to prevent regular
update messages from inappropriately reinstating a route that
may not be up. When a router goes down, neighboring routers
detect this via the lack of regularly scheduled update
messages. Split horizons
Split horizons are derived
from the premise that it is usually not useful to send
information about a route back in the direction from which it
came. The split horizon rule helps prevent routing loops.
Poison reverse updates
Split horizons prevent
routing loops between adjacent routers, but poison reverse
updates are necessary to defeat larger routing loops. Generally
speaking, increases in routing metrics indicate routing loops.
Poison reverse updates then are sent to remove the route and
place it in holddown. With IGRP, poison reverse updates are
sent only if a route metric has increased by a factor of 1.1 or
greater. IGRP also maintains a number of timers and variables
containing time intervals. These include an update timer, an
invalid timer, a holddown timer, and a flush timer. The update
timer specifies how frequently routing update messages should
be sent. The IGRP default for this variable is 90 seconds. The
invalid timer specifies how long a router should wait in the
absence of routing-update messages about a specific route
before declaring that route invalid. The IGRP default for this
variable is three times the update period. The holddown timer
specifies the amount of time for which information about poorer
routes is ignored. The IGRP default for this variable is three
times the update timer period plus 10 seconds. Finally, the
flush timer indicates how much time should pass before a route
is flushed from the routing table. The IGRP default is seven
times the routing update timer. Today, IGRP is showing its age,
it lacks support for variable length subnet masks (VLSM).
Rather than develop an IGRP version 2 to correct this problem,
Cisco has built upon IGRP's legacy of success with Enhanced
IGRP. Web Links An Introduction to IGRP
http://www.cisco.com/en/US/tech/tk365/
tk352/technologies_white_paper09186a0
0800c8ae1.shtml
Content 7.3 IGRP
7.3.5 Configuring IGRP To configure the IGRP
routing process, use the router igrp configuration
command. To shut down an IGRP routing process, use the
no form of this command. RouterA(config)#router
igrp as-number
RouterA(config)#no router
igrp as-number The Autonomous System number is one
that identifies the IGRP process. It is also used to tag the
routing information. To specify a list of networks for IGRP
routing processes, use the network router configuration
command. To remove an entry, use the no form of the
command. Figure is an example of how to configure IGRP using AS
101. Lab Activity Lab Exercise: Configuring IGRP This
lab is to setup an IP addressing scheme using class C
networks. Lab Activity e-Lab Activity: Configuring
IGRP In this lab, the students will learn to configure IGRP.
Web Links Configuring IGRP
http://www.cisco.com/en/US/products/sw/ iosswrel/ps1831/
products_configuration_guide_ chapter09186a00800d97f6.html
Content 7.3 IGRP 7.3.6
Migrating RIP to IGRP With the creation of IGRP in the
early 1980s, Cisco Systems was the first company to solve the
problems associated with using RIP to route datagrams between
interior routers. IGRP determines the best path through the
internetwork by examining the bandwidth and delay of the
networks between routers. IGRP converges faster than RIP,
thereby avoiding the routing loops caused by disagreement over
the next routing hop to be taken. Further, IGRP does not share
the hop count limitation of RIP. As a result of this and other
improvements over RIP, IGRP enabled many large, complex,
topologically diverse internetworks to be deployed.These are
the steps to follow to convert from RIP to IGRP.
- Verify existing routing protocol (RIP) on the routers to be
converted.
- Configure IGRP on RouterA and RouterB
- Enter show ip protocols on RouterA and
RouterB
- Enter show ip route on RouterA and
RouterB
Lab Activity Lab Exercise: Default
Routing with RIP and IGRP This lab is to configure a default