Introduction to Dynamic Routing Protocol ppsx

Introduction to Dynamic
Routin
g
Protocol
g
Routing Protocols and Concepts – Chapter 3
© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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Objectives
 D
esc
ri
be
th
e
r
o
l
e

o
f
dy
n
a
mi
c
r
ou

tin
g

p
r
o
t
oco
l
s

a
n
d

escbe eoeodya cou gpoocosad
place these protocols in the context of modern
network design.
 Identify several ways to classify routing protocols.

Describe how metrics are used by routing protocols
Describe

how

metrics

are

used

by

routing

protocols

and identify the metric types used by dynamic routing
protocols.
 Determine the administrative distance of a route and
describe its importance in the routing process.
 Identify the different elements of the routing table.
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Dynamic Routing Protocols
 D
y
n
a
mi
c
r
ou
tin
g

p
r

o
t
oco
l
s

a
r
e

usua
ll
y

y a c ou g p o oco s a e usua y
used in larger networks to ease the
administrative and operational overhead
fi ltti t
o
f
us
i
ng on
l
y s
t
a
ti
c rou
t

es.
 Typically, a network uses a combination
f b th d i ti t l d
o
f

b
o
th
a
d
ynam
i
c rou
ti
ng pro
t
oco
l
an
d

static routes.
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The Evolution of Dynamic Routing Protocols
 One of the earliest routing protocols was Routing Information Protocol (RIP).
RIP h l d i t i RIP 2 H

–
RIP

h
as evo
l
ve
d

i
n
t
o a newer vers
i
on
RIP
v
2
.
H
owever,
–The newer version of RIP still does not scale to larger network implementations.
 To address the needs of larger networks, two advanced routing protocols were
developed: Open Shortest Path First (OSPF) and Intermediate System-to-
It dit S t (IS
IS)
I
n
t
erme

di
a
t
e
S
ys
t
em
(IS
-
IS)
.
 Cisco developed Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP
(EIGRP), which also scales well in larger network implementations.

Additionally there was the need to interconnect different internetworks and provide

Additionally
,
there

was

the

need

to

interconnect

different

internetworks

and

provide

routing among them. Border Gateway Routing (BGP) protocol is now used between
ISPs as well as between ISPs and their larger private clients to exchange routing
information.

With the advent of numerous consumer devices using IP the IPv4 addressing space

With

the

advent

of

numerous

consumer

devices

using

IP
,
the

IPv4

addressing

space

is nearly exhausted. Thus IPv6 has emerged. To support the communication based
on IPv6, newer versions of the IP routing protocols have been developed (see the
IPv6 row in the table).
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Dynamic Routing Protocols
 Function(s) of Dynamic Routing Protocols:
-Dynamically share information between routers.
-Automatically update routing table when topology changes.
-
Determine best path to a destination
Determine

best

path

to

a

destination
.
–Compared to static routing, dynamic routing protocols require less administrative
overhead.
•
However, the expense of using dynamic routing protocols is dedicating part of a router
'
s
However,

the

expense

of

using

dynamic

routing

protocols

is

dedicating

part

of

a

router s

resources for protocol operation including CPU time and network link bandwidth.
– One of the primary benefits to using a dynamic routing protocol is that routers
exchange routing information whenever there is a topology change. This exchange
ll t t t ti ll l b t t k d l t fi d lt t
a
ll
ows rou
t
ers
t
o au
t
oma
ti
ca
ll
y
l
earn a
b

ou
t
new ne
t
wor
k
s an
d
a
l
so
t
o
fi
n
d
a
lt
erna
t
e
paths when there is a link failure to a current network.
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Dynamic Routing Protocols
 D
esp
it

e
th
e

be
n
e
fit
s

o
f
dy
n
a
mi
c
r
ou
tin
g,

s
t
a
ti
c
r
ou
tin

g

s
till
esp e e be e s o dy a c ou g, s a c ou g s
has its place.

There are times when static routing is more appropriate and
There

are

times

when

static

routing

is

more

appropriate

and

other times when dynamic routing is the better choice.


More often than not you will find a combination of both
More

often

than

not
,
you

will

find

a

combination

of

both

types of routing in any network that has a moderate level of
complexity.
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Dynamic Routing Protocols

 A routin
g

p
rotocol
gp
–is a set of processes, algorithms, and messages that are used to
exchange routing information and populate the routing table with the
routin
g

p
rotocol's choice of best
p
aths
gp p
 The purpose of a dynamic routing protocol is to:
-Discover remote networks
-Maintaining up-to-date routing information
-Choosing the best path to destination networks
Abilit t
fi d b t th
if th t th i l il bl
-
Abilit
y
t
o
fi
n

d
a new
b
es
t
pa
th
if

th
e curren
t
pa
th

i
s no
l
onger ava
il
a
bl
e
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Dynamic Routing Protocols
Dynamic

Routing

Protocols
 Components of a routing protocol
–
Data structures
Data

structures
•Some routing protocols use tables and/or databases for its operations.
This information is kept in RAM
Al ith
–
Al
gor
ith
m
•Algorithm is a finite list of steps used in accomplishing a task
•
Algorithms are used for facilitating routing information and best path
Algorithms

are

used

for

facilitating

routing

information

and

best

path

determination
–Routing protocol messages
Th f
di i i hb
d
hf
•
Th
ese are messages
f
or
di
scover
i
ng ne
i
g
hb
ors an
d

exc
h
ange o
f

routing information , and other tasks to learn and maintain accurate
information about the network.
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Dynamic Routing Protocol Operation
Dynamic

Routing

Protocol

Operation
 All routing protocols have the same purpose - to learn about remote networks
and to quickly adapt whenever there is a change in the topology.
 The method that a routing protocol uses to accomplish this depends upon the
algorithm it uses and the operational characteristics of that protocol.

In general the operations of a dynamic routing protocol can be described as

In

general
,

the

operations

of

a

dynamic

routing

protocol

can

be

described

as

follows:
–The router sends and receives routing messages on its interfaces.
–
The router shares routing messages and routing information with other routers that
are using the same routing protocol.
–Routers exchange routing information to learn about remote networks.
–
When a router detects a topology change the routing protocol can advertise this

change to other routers.
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Dynamic Routing Protocols
 Advantages of static routing
It b k lti l
 Advantages of dynamic routing
-
It
can
b
ac
k
up mu
lti
p
l
e
interfaces/networks on a router
-Minimal CPU processing
Easier for administrator to
-Administrator has less work
maintaining the configuration when
adding or deleting networks.
-
Easier

for

administrator

to

understand
-Easy to configure
-
No extra resources are needed
-Protocols automatically react to the
topology changes.
-Confi
g
uration is less erro
r
-prone.
No

extra

resources

are

needed
-More secure
 Disadvantages of static routing
g
-More scalable, growing the network
usually does not present a problem


Disadvantages
of
dynamic routing
-Network changes require manual
reconfiguration
-Configuration and maintenance is
time
-
consuming

Disadvantages
of

dynamic

routing
-Router resources are used (CPU
cycles, memory and link bandwidth).
time
consuming
-Does not scale well in large
topologies
-Confi
g
uration is erro
r
-
p
rone

,

-More administrator knowledge is
required for configuration,
verification, and troubleshooting.
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g
p,
especially in large networks
Dynamic Routing Protocols
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Classifying Routing Protocols
 D
y
namic routin
g

p
rotocols are
g
rou
p
ed accordin
g

to
ygp
gp g
characteristics. Examples include:
-RIP
IGRP
-
IGRP
-EIGRP
OSPF
-
OSPF
-IS-IS
-
BGP
BGP
 Autonomous System is a group of routers under the control of
a sin
g
le authorit
y
.
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gy
Classifying Routing Protocols
 Dynamic routing protocols:
–RIP

•A distance vector interior routing protocol
–
IGRP
•The distance vector interior routing
developed by Cisco (deprecated from 12.2
IOS and later)
–
EIGRP
•The advanced distance vector interior
routing protocol developed by Cisco
OSPF
–
OSPF
•A link-state interior routing protocol
–IS-IS
•A link-state interior routing protocol
–BGP
•
A

p
ath vector exterior routin
g

p
rotocol
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pgp
Classifying Routing Protocols
Classifying

Routing

Protocols
 An autonomous system (AS) - otherwise known as a
routing domain - is a collection of routers under a
common administration
.
common

administration
.
 Because the Internet is based on the ASs concept, two
types of routing protocols are required: interior and
exterior routing protocols.
Interior Gateway Protocols
(IGP)
-
Interior

Gateway

Protocols
(IGP)
•are used for intra-autonomous system routing - routing
inside an autonomous system
•IGPs are used for routing within a routing domain, those

networks within the control of a sin
g
le or
g
anization.
gg
–An autonomous system is commonly comprised of many
individual networks belonging to companies, schools, and
other institutions.
• IGPs for IP include RIP, IGRP, EIGRP, OSPF, and IS-IS
Exterior Gateway Protocols
(EGP)
-
Exterior

Gateway

Protocols
(EGP)
•are used for inter-autonomous system routing - routing
between autonomous systems that are under the control
of different administrations
•
At the ISP level, there are often more important issues
At

the

ISP

level,

there

are

often

more

important

issues

than just choosing the fastest path.
•BGP is typically used between ISPs and sometimes
between a company and an ISP
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At t
A
u
t
onomous sys
t
ems
 An autonomous system (AS) is a collection of
networks under a common administration

networks

under

a

common

administration

sharing a common routing strategy.
To the outside world, an AS is viewed as a single
entity. The AS may be run by one or more
operators while presenting a consistent view of
routing to the external world.
 The American Registry of Internet Numbers
(ARIN), a service provider, or an administrator
assi
g
ns an identif
y
in
g
number to each AS. This
gyg
autonomous system number is a 16 bit number.
Routing protocols, such as Cisco’s IGRP,
require assignment of a unique, autonomous
system number.
American Registry for Internet Numbers

/>A t S t b (ASN) id
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A
u
t
onomous
S
ys
t
em num
b
er
(ASN)
resource gu
id
e
/>IS-IS
Autonomous systems
Autonomous

systems

 Cisco system AS number:
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Autonomous systems
Autonomous

systems

 />RFC 1930
 AS just like IP, it needs
to apply from ARIN or
the appropriate region
and be unique on the
it t
i
n
t
erne
t
.
 The Internet Assigned
Numbers Authority
(IANA) has reserved the
following block of AS
numbers for private use
(not to be advertised on
the global Internet):
64512 through 65535
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Classifying Routing Protocols

Classifying

Routing

Protocols
 IGP: Comparison of Distance Vector & Link
State Routing Protocols
Distance vector
– routes are advertised as vectors of distance &
direction.
•Distance is defined in terms of a metric such as hop
count (RIP)
•Direction is simply the next-hop router or exit
interface
interface
•Distance vector protocols typically use the Bellman-
Ford algorithm for the best path route determination
–
incomplete view of network topology
incomplete

view

of

network

topology
.
•Distance vector protocols use routers as sign posts

along the path to the final destination.
•Distance vector routin
g

p
rotocols do not have an
gp
actual map of the network topology
– Generally, periodic updates.
•Some distance vector protocols periodically send
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complete routing tables to all connected neighbors.
Classifying Routing Protocols
Classifying

Routing

Protocols
 IGP: Comparison of Distance Vector &
Link State
Routing Protocols
Link

State
Routing

Protocols

Link state
–
com
p
lete view of network to
p
olo
gy
is created.
ppgy
•The sign posts along the way from source to
destination are not necessary, because all link-
state routers are using an identical
"
map
"
of the
state

routers

are

using

an

identical

map

of

the

network.
– updates are not periodic.
•After the network has converged, a link-state
update only sent when there is a change in the
topology.
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Classifying Routing Protocols
Classifying

Routing

Protocols
 Comparison of Distance Vector & Link State Routing Protocols
Di t t t l k

Di
s
t
ance vec
t
or pro
t

oco
l
s wor
k

best in situations where:
–
The network is sim
p
le and flat
 Link-state protocols work best in
situations where:
–
The network desi
g
n is hierarchical
,

p
and does not require a special
hierarchical design.
–
The administrators do not have
g,
usually occurring in large networks.
–The administrators have a good
knowledge of the implemented link
-
enough knowledge to configure
and troubleshoot link-state

protocols.
knowledge

of

the

implemented

link
state routing protocol.
–Fast convergence of the network is
crucial
–Specific types of networks, such
as hub-and-spoke networks, are
being implemented.
crucial
.
–Worst-case convergence times
in a network are not a concern.
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Classifying Routing Protocols
 Classful routing protocols
–
Do NOT send
subnet mask in routing updates
Do

NOT

send
subnet

mask

in

routing

updates
,
–Do NOT support VLSM,
–Classful routing protocols cannot be used when
a network is subnetted usin
g
more than one
g
subnet mask,
• Tony: This does not mean you can not
subnet the clasasfull network. You can still
subnet it but can only do it once and all
subnet

it
,
but

can

only

do

it

once

and

all

network needs to have the identical mask.
–Routing protocols such as RIPv1 and IGRP.
 Classless routin
g

p
rotocols
gp
–Do send subnet mask in routing updates.
–support variable length subnet masks (VLSM).
•In the fi
g
ure
,
the classless version of the network is
g,

using both /30 and /27 masks in the same topology.
•Tony: It means you can create the network
with all different sizes of subnets. They don’t
need to have the same mask.
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need

to

have

the

same

mask.

•Classless routing protocols are RIPv2, EIGRP,
OSPF, IS-IS, BGP.
Classifying Routing Protocols

Co
nv
e
r
ge
n

ce
i
s

de
fin
ed

as
wh
e
n
a
ll r
ou
t
e
r
s
’ r
ou
tin
g

Co e ge ce
sdeedas eaouesoug
tables are at a state of consistency
– The network has converged when all routers have complete and
f
accurate in

f
ormation about the network
 Convergence time is the time it takes routers to share
i f ti l l t b t th d d t th i ti
i
n
f
orma
ti
on, ca
l
cu
l
a
t
e
b
es
t
pa
th
s, an
d
up
d
a
t
e
th
e

i
r rou
ti
ng
tables.
Rti t l b tdb d

R
ou
ti
ng pro
t
oco
l
s can
b
e ra
t
e
d

b
ase
d
on
the speed to convergence; the faster the
convergence, the better the routing
tl
pro
t

oco
l
.
–RIP and IGRP are slow to converge
–
EIGRP and OSPF are faster to converge.
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EIGRP

and

OSPF

are

faster

to

converge.

Routing Protocols Metrics
 To select the best path, the routing
lbbl l d
protoco
l
must

b
e a
bl
e to eva
l
uate an
d

differentiate between the available paths.
For this purpose a metric is used.
 Metric
–A value used by a routing protocol to
determine which routes are better than others.
 Each routing protocol uses its own metric.
–RIP uses hop count,
•
The hop count refers to the number of routers
The

hop

count

refers

to

the

number

of

routers

a packet must cross to reach the destination
network.
•For R3 in the figure, network 172.16.3.0 is two
hops or two routers away
hops
,
or

two

routers

away
.
–EIGRP uses a combination of bandwidth and
delay,
–
OSPF
uses
bandwidth (cost)
.
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OSPF
uses

bandwidth

(cost)
.

Routing Protocols Metrics
 Metrics used in IP routing protocols
–
Bandwidth
•Influences path selection by preferring the path
with the highest bandwidth
–
Cost
Cost
•A value determined either by the IOS or by the
network administrator to indicate preference for a
route. Cost can represent a metric, a combination
of metrics or a
p
olic
y
.
RIP
OSPF
py
–Delay
•Considers the time a packet takes to traverse a

path
Hop count
RIP
–
Hop

count
•A simple metric that counts the number of routers
a packet must traverse
–
Load
•Considers the traffic utilization of a certain link
–Reliability
•Assesses the probability of a link failure,
calculated from the interface error count or
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calculated

from

the

interface

error

count

or

previous link failures
Routing Protocols Metrics
Routing

Protocols

Metrics
 The Metric Field in the Routing Table
 Metric used for each routing protocol
-RIP - hop count
-IGRP & EIGRP - Bandwidth (used by
default), Delay (used by default), Load,
Reliability
Reliability
-IS-IS & OSPF – Cost, Bandwidth
(Cisco’s implementation)

Refer to the example in the figure The
Refer

to

the

example

in

the

figure

The

routers are using the RIP routing
protocol.
–The metric associated with a certain
tbbtidith
rou
t
e can
b
e
b
es
t
v
i
ewe
d
us
i
ng
th
e
show ip route command.
–The metric value is the second value in

the brackets for a routing table entry.
–In the figure, R2 has a route to the
192.168.8.0/24 network that is 2 hops
away.
•
R 192 168 8 0/24 [120/2] via
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R

192
.
168
.
8
.
0/24

[120/2]

via

192.168.4.1, 00:00:26, Serial0/0/1