Part 2 of 2: Distance Vector Routing and IGRP

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Ch.16/Mod.7


Distance Vector Routing
Protocols

Part 2 of 2: Distance Vector Routing and
IGRP

IGRP Features


IGRP is a distance vector routing protocol developed by Cisco.


IGRP sends routing updates at 90 second intervals, advertising
networks for a particular autonomous system.


Key design characteristics of IGRP are a follows:


The versatility to automatically handle indefinite, complex
topologies



The flexibility needed to segment with different bandwidth and
delay characteristics



Scalability for functioning in very large networks


IGRP Features


By default, the IGRP routing protocol uses bandwidth and delay as
metrics.


Additionally, IGRP can be configured to use a combination of variables
to determine a composite metric.


Those variables include:


Bandwidth


Delay


Load


Reliability

IGRP Metrics

IGRP Metrics


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



NO…

MTU



The Maximum Transmission Unit value of the path.
MTU has never been used by IGRP or EIGRP as a routing
metric.



IGRP has an
administrative distance
of

100
, more “trustworthy” than
RIP at 120.


This means a Cisco router will prefer an IGRP learned route over a RIP
learned route to the same network.

Administrative Distances

IGRP Metrics

IGRP 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.”


Clarification


IGRP also advertises three types of routes:


interior, system, and exterior.


Interior routes

are routes between subnets in the network attached to a
router interface.


If the network attached to a router is not subnetted, IGRP does not
advertise interior routes.


IGRP 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.”

IGRP Routes


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.”

IGRP Timers


IGRP has a number of features that are designed to enhance its
stability, such as:



Holddowns



Split horizons



Poison reverse updates


IGRP
Timers


The
update timer

specifies how frequently routing update
messages should be sent.


The IGRP default for this variable is
90 seconds
.

Update timer

IGRP
Timers


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 (unreachable),
but still in the routing table
.


The IGRP default for this variable is
three times the update period
or
270 seconds
.


Then placed in the
holddown
state.


“If I haven’t heard from you in 270 seconds, I am considering this route
as unreachable, I will start the holddown timer, but I will keep it in the
routing table until the flush timer expires.”

Invalid timer

IGRP
Timers


The
holddown timer

specifies the amount of time for which
information about poorer routes are ignored.


Zinin: “Holddown specifies the number of seconds that a route must
spend in holddown state after expiration of the Invalid Timer.”


The IGRP default for this variable is
three times the update timer
period plus 10 seconds = 280 seconds
.


The original route is still in the routing table but marked as
unreachable, until the flush timer expires.

Holddown
timer

IGRP
Timers


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

or
630 seconds
.


Zinin: “Flush specifies the number of seconds that a route must remain in the
routing table in the garbage collection state after it exits the holddown state.”


Each time an
update is received

the
invalid

and
flush

timers

are
reset
.


If the
invalid timer

expires before another update is heard, the route is marked
as
unreachable
, but remains in the routing table.


If the
flush timer

then expires before another update is heard, the route will be
deleted from the routing table
.

Flush timer

IGRP
Timers


All timers begin at the same time.


Update timer = 90 seconds


Invalid timer = 270 seconds


Holddown timer = 280 seconds


Flush timer = 630 seconds


Today, IGRP is showing its age, it lacks support for variable length
subnet masks (VLSM).


Enhanced IGRP (EIGRP) supports VLSM.

Configuring IGRP


Same network commands as RIP.


IGRP “AS” number must be the same on all routers.

Configuring IGRP

timers basic (IGRP)


To adjust Interior Gateway Routing Protocol (IGRP)
network timers, use the
timers basic

router configuration
command. To restore the default timers, use the
no

form of
this command.



Router(config
-
router)#
router igrp 100

Router(config
-
router)#
timers basic
update invalid holddown
flush

[
sleeptime
]

Router(config
-
router)#

no timers basic

Migrating from RIP to IGRP

Router(config)#
router rip

Router(config
-
router)#
network 172.16.0.0

Router(config
-
router)#
network 192.168.1.0

Router(config
-
router)#
exit

Router(config)#
router igrp 10

Router(config
-
router)#
network 172.16.0.0

Router(config
-
router)#
network 192.168.1.0

Router(config
-
router)#
exit

Router(config)#
no router rip


Enable IGRP


Suggestion: Remove RIP configuration from routers even
though the administrative distance will prefer RIP

Verifying IGRP

Verifying IGRP

Verifying IGRP

Verifying IGRP

Verifying IGRP

Troubleshooting IGRP

Troubleshooting IGRP

Troubleshooting IGRP

Summary

But there is still more!

IGRP Metric Information

Router> show interfaces s1/0

Serial1/0 is up, line protocol is up


Hardware is QUICC Serial


Description: Out to VERIO


Internet address is 207.21.113.186/30


MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,


rely 255/255, load 246/255


Encapsulation PPP, loopback not set


Keepalive set (10 sec)

<output omitted>

bandwidth

delay

reliability

load

Metric Calculation

Displaying Interface Values

Router> show interface s0/0

Serial0/0 is up, line protocol is up


Hardware is QUICC Serial


Description: Out to VERIO


Internet address is 207.21.113.186/30


MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,


rely 255/255, load 246/255


Encapsulation PPP, loopback not set


Keepalive set (10 sec)

<output omitted>

Bandwidth

Delay

Reliability

Load

Routing Table Metric


Default: Slowest of bandwidth plus the sum of the delays of
all outgoing interfaces from “this router” to the destination
network.

Bandwidth


Expressed in
kilobits

(
show interface
)


This is a
static number

and used for metric calculations only.


Does not necessarily reflect the actual bandwidth of the link.


It is an
information parameter only
.


You cannot adjust the actual bandwidth on an interface with this command.


Use the
show interface

command to display the
raw value


The default values:


Default bandwidth of a Cisco interface depends on the type of interface.



Default bandwidth of a Cisco
serial interface is 1544 kilobits

or 1,544,000
bps (T1), whether that interface is attached to a T1 line (1.544 Mbps) or a 56K
line.



IGRP metric uses the
slowest bandwidth

of all of the outbound interfaces to
the destination network.

Metric Calculation

Changing the bandwidth informational parameter:


The bandwidth can be changed using:


Router(config
-
if)#
bandwidth

kilobits


To restore the default value:


Router(config
-
if)#
no bandwidth



Metric Calculation

Delay


Like bandwidth, delay it is a
static number
.


Expressed in
microseconds
,
millionths of a second



(Uses the Greek letter mu with an S,

S, NOT “ms” which is millisecond or
thousandths of a second
)


Use the
show interface

command to display the
raw value


It is an
information parameter only
.


The default values:


The default delay value of a Cisco interface depends upon the
type of
interface
.



Default delay of a Cisco
serial interface

is
20,000 microseconds
, that of a T1
line.



IGRP metric uses the
sum of all of the delays

of all of the outbound interfaces
to the destination network.

Metric Calculation

Changing the delay informational parameter:

The delay can be changed using:


Router(config
-
if)#
delay

tens
-
of
-


S



(microseconds)



Example of changing the delay on a serial interface to 30,000
microseconds
:


Router(config
-
if)#
delay

3000


To restore the 20,000 microsecond default value:


Router(config
-
if)#
no delay


Metric Calculation

IGRP


bandwidth = (10,000,000/
bandwidth
)



delay =
delay
/10


Metric Calculation

IGRP Metrics


Values displayed in show interface
commands and sent in routing updates.

Router> show interfaces s1/0

Serial1/0 is up, line protocol is up


Hardware is QUICC Serial


Description: Out to VERIO


Internet address is 207.21.113.186/30


MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,


rely 255/255, load 246/255


Encapsulation PPP, loopback not set


Keepalive set (10 sec)

<output omitted>

bandwidth

delay

reliability

load

Metric Calculation

From Casablanca to 172.20.40.0/24

Using
BW
IGRP

and
DLY
IGRP

to calculate the IGRP metric:


The slowest bandwidth has the highest
BW
IGRP

value.


IGRP metric


=
highest

BW
IGRP

+ total of the
DLY
IGRP


= 19,531 + (100 + 2,000 + 2,000 + 100)


= 19,531 + 4,200


=
23,731

From Casablanca to 172.20.40.0/24

Calculating the IGRP Metric

Using the

Raw Values:

Bandwidth and Delay

From Casablanca to 172.20.40.0/24

So how is Bandwidth, BW
IGRP
, calculated?


The
bandwidth

metric is calculated by taking
10
7

(10,000,000)
and dividing it

by

the
slowest bandwidth metric along the route to
the destination
.


This is known as taking the inverse of the bandwidth scaled by a
factor of 10
7
(10,000,000)


The
lowest bandwidth

on the route is 512K or
512

(measured in
kilobits), the outgoing interface of the Quebec router.


Divide 10,000,000 by 512

and you get the bandwidth!


Bandwidth

= 10,000,000/512


=
19,531

Which is the lowest
BW
IGRP
along the route

Calculating Bandwidth

So how is Delay, DLY
IGRP
, calculated?


Delay

is the
total sum of delays on the outgoing interfaces
, in 10
-
microsecond units


The sum of the delays on each of the
outgoing interfaces

between
Casablanca and Yalta, from 172.20.1.0/24 through 172.20.40.0/24 is:


1,000 (Casablanca) + 20,000 (Teheran) + 20,000 (Quebec) + 1,000
(Yalta) =
42,000


We need this in 10
-
microsecond units:

= (1,000/10)+(20,000/10) + (20,000/10) + (1,000/10)

= 100 + 2,000 + 2,000 + 100

or

= (1,000 + 20,000 + 20,000 +1,000) / 10


In either case the total sum is:


Delay = 4,200

Which is the total of the

DLY
IGRP
, the total Delays along the route!

Calculating Delay

IGRP metric = Bandwidth + Delay


IGRP metric = 19,531 + 4,200


=
23,731


IF

we were using
RIP,

the RIP metric would be
3 hops
.


Slowest Bandwidth + Sum of Delays

Casablanca# show ip route 172.20.40.0

Known via igrp 1, distance 100, metric 23,731



172.20.1.2, from 172.20.1.2 on Ethernet 0

Route metric is 23,731

Total delay is 42,000 microseconds,

minimum bandwidth is 512 Kbit

...



Not to be redundant, but if we were using RIP, the
RIP metric would be 3 hops.


show ip route 172.20.40.0


Ch.16/Mod. 7


Distance Vector Routing
Protocols

Part 2 of 2: Distance Vector Routing and
IGRP