Routing Protocols and Concepts – Chapter 4 docx

Distance Vector Routing
Protocols
Routing Protocols and Concepts – Chapter 4
© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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Objectives
 Identif
y
the characteristics of distance vector routin
g

p
rotocols.
ygp
 Describe the network discovery process of distance vector
routing protocols using Routing Information Protocol (RIP).
 Describe the processes to maintain accurate routing tables used
by distance vector routing protocols.
Id tif th diti l di t ti l d l i th

Id
en
tif
y
th
e con
diti
ons
l

ea
di
ng
t
o a rou
ti
ng
l
oop an
d
exp
l
a
i
n
th
e
implications for router performance.

Recognize that distance vector routing protocols are in use today
Recognize

that

distance

vector

routing

protocols

are

in

use

today

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Distance Vector Routing Protocols
 Dynamic routing protocols help the network administrator overcome the time-
consuming and exacting process of configuring and maintaining
static routes
consuming

and

exacting

process

of

configuring

and

maintaining

static

routes
.
 Examples of Distance Vector routing protocols:
Routing Information Protocol (RIP)
RFC 1058
–
RFC

1058
.
–Hop count is used as the metric for path selection.
–If the hop count for a network is greater than 15, RIP cannot supply a route to that
network.
R ti d t b d t lti t 30 d b d f lt
–
R
ou
ti
ng up
d
a
t
es are
b

roa
d
cas
t
or mu
lti
cas
t
every
30
secon
d
s,
b
y
d
e
f
au
lt
.
Interior Gateway Routing Protocol (IGRP)
–proprietary protocol developed by Cisco.
–
Bandwidth, delay, load and reliability are used to create a composite metric.
Bandwidth,

delay,

load

and

reliability

are

used

to

create

a

composite

metric.

–Routing updates are broadcast every 90 seconds, by default.
–IGRP is the predecessor of EIGRP and is now obsolete.
Enhanced Interior Gatewa
y
Routin
g
Protocol
(
EIGRP
)
yg

()
–Cisco proprietary distance vector routing protocol.
–It can perform unequal cost load balancing.
–It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path.
–
There are no periodic updates as with RIP and IGRP Routing updates are sent only
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–
There

are

no

periodic

updates

as

with

RIP

and

IGRP

.
Routing

updates

are

sent

only

when there is a change in the topology.
Distance Vector Routing Protocols

The Meaning of Distance Vector
:
The

Meaning

of

Distance

Vector
:
–A router using distance vector routing protocols knows 2 things:

Distance
to final destination


Distance

to

final

destination
The distance or how far it is to the destination network

Vector or direction
traffic should be directed

Vector
,
or

direction
,
traffic

should

be

directed
The direction or interface in which packets should be forwarded
For example, in the figure,
R1 knows that the distance
to reach network

172.16.3.0/24 is 1 hop and
that the direction is out the
i t f S0/0/0 t d R2
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i
n
t
er
f
ace
S0/0/0

t
owar
d

R2
.
Distance Vector Routing Protocols
Distance

Vector

Routing

Protocols
 Characteristics of Distance Vector routing protocols:

 Periodic u
p
dates
p
•Periodic Updates sent at regular intervals (30 seconds for
RIP). Even if the topology has not changed in several days,
 Neighbors
The router is only aware of the network addresses of its
own interfaces and the remote network addresses it can
reach through its neighbors.
It has no broader knowledge of the network topology
 Broadcast updates
Broadcast Updates are sent to 255.255.255.255.
Some distance vector routing protocols use multicast
addresses instead of broadcast addresses
addresses

instead

of

broadcast

addresses
.
 Entire routing table is included with routing update
Entire Routing Table Updates are sent, with some
e
x
cept

i
o
n
s

to

be

d
i
scussed
l
ate
r
,

pe
ri
od
i
ca
ll
y

to

a
ll
exceptions

to

be

discussed

later,

periodically

to

all

neighbors.
Neighbors receiving these updates must process the entire
update to find pertinent information and discard the rest.

Some distance vector routing protocols like EIGRP do not
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
Some

distance

vector

routing

protocols

like

EIGRP

do

not

send periodic routing table updates.
Distance Vector Routing Protocols
Distance

Vector

Routing

Protocols
 Routing Protocol Algorithm:
Th l ith i d t l l t th b t th d th d
–
Th
e a
l
gor
ith

m
i
s use
d

t
o ca
l
cu
l
a
t
e
th
e
b
es
t
pa
th
s an
d

th
en sen
d

that information to the neighbors.
–
Different routin

g

p
rotocols use different al
g
orithms to install routes
gp g
in the routing table, send updates to neighbors, and make path
determination decisions.
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Distance Vector Routing Protocols
Routing Protocol Characteristics
Criteria used to compare routing protocols
includes
–
Criteria

used

to

compare

routing

protocols
includes

Time to convergence
Time to convergence defines how quickly the routers in the network topology share
routing information and reach a state of consistent knowledge.
The faster the convergence, the more preferable the protocol.
Scalability
Scalability defines how large a network can become based on the routing protocol that is
deployed
deployed
.
The larger the network is, the more scalable the routing protocol needs to be.
Resource usage
Resource usa
g
e includes the re
q
uirements of a routin
g

p
rotocol such as memor
y
s
p
ace
,

gq gp yp,
CPU utilization, and link bandwidth utilization.
Higher resource requirements necessitate more powerful hardware to support the routing
protocol operation

Classless
(
Use of VLSM
)
or Classful
()
Classless routing protocols include the subnet mask in the updates.
This feature supports the use of Variable Length Subnet Masking (VLSM) and better route
summarization.

Implementation & maintenance
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
Implementation

&

maintenance
Implementation and maintenance describes the level of knowledge that is required for a
network administrator to implement and maintain the network based on the routing protocol
deployed.
Distance Vector Routing Protocols
Distance

Vector

Routing

Protocols
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Network Discovery

Router initial start up
(Cold Starts)
Cold Starts

Router

initial

start

up
(Cold

Starts)
When a router cold starts or powers up, it knows nothing about the
network topology. It does not even know that there are devices on
the other end of its links. The only information that a router has is
from its own saved configuration file stored in NVRAM.
Initial network discovery
-
Initial

network

discovery
Directly connected networks are initially placed in
routing table
routing

table

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Network Discovery
I iti l E h
Network

Discovery
 Initial Exchange of Routing Information
–
If
a routing protocol
is
configured
then
I
n
iti
a
l

E
xc
h
ange
If

a

routing

protocol
is

configured
then
•Routers will exchange routing information
•Initially, these updates only include information
about their directly connected networks.
 Routing updates received from other routers
–Router checks update for new information
•If there is new information:
–Metric is updated
–New information is stored in routing table
 After this first round of update exchanges, each
tk btth td t kfthi
rou
t
er
k

nows a
b
ou
t

th
e connec
t
e
d
ne
t
wor
k
s o
f

th
e
i
r
directly connected neighbors.
 However, did you notice that R1 does not yet know
about 10400andthatR3does not yet know about
about

10
.
4
.

0
.
0

and

that

R3

does

not

yet

know

about

10.1.0.0?
–Full knowledge and a converged network will not take
place until there is another exchange of routing
information
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information
.

Network Discovery
Network

Discovery
 Next Update of Routing Information
At thi i t th t h k l d b t
Next Update
–
At

thi
s po
i
n
t

th
e rou
t
ers
h
ave
k
now
l
e
d
ge a
b
ou

t

their own directly connected networks and
about the connected networks of their
immediate neighbors
immediate

neighbors
.
–Continuing the journey toward convergence,
the routers exchange the next round of periodic
updates Each router again checks the updates
updates
.
Each

router

again

checks

the

updates

for new information.
 Routing updates received from other routers
–Router checks update for new information
•If there is new information:

–
Metric is updated
–New information is stored in routing
table
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Network Discovery
Network

Discovery
 Distance vector routing protocols
Split horizon
typically implement a technique
known as split horizon.
–Split horizon prevents information
from being sent out the same
interface from which it was
interface

from

which

it

was

received.

For example R2 would not send
–
For

example
,
R2

would

not

send

an update out Serial 0/0/0
containin
g
the network 10.1.0.0
g
because R2 learned about that
network through Serial 0/0/0.
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Network Discovery
Network

Discovery
 Exchange of Routing Information

Next Update
–
Router convergence is reached when
•All routing tables in the network contain the same network
information
information
,
•[Tony]: The above statement is trying to tell you, the routing tables
contains the same network information, BUT, each router has it’s own
ii fh i bl
var
i
at
i
on o
f
t
h
e rout
i
ng ta
bl
e.
–Routers continue to exchange routing information
-If no new information is found then Convergence is
reached
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Network Discovery and convergence

The amount of time it takes for a network to converge is
The

amount

of

time

it

takes

for

a

network

to

converge

is

directly proportional to the size of that network.
 Convergence must be reached before a network is considered
completely operable

 Speed of achieving convergence consists of 2 interdependent
categories
categories
–How quickly the routers propagate a change in the topology in a
routing update to its neighbors
–The speed of calculating best path routes using the new routing
information collected
4
5
For example: It takes five rounds
of periodic update intervals
before most of the branch
routers in Regions 1 2 and 3
2
3
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routers

in

Regions

1
,
2
,
and

3

learn about the new routes
advertised by B2-R4.
1
Routing Table Maintenance

Periodic Updates
: RIPv1 & RIPv2

Periodic

Updates
:

RIPv1

&

RIPv2
–These are time intervals in which a router sends out its entire routing
table.
•RIPv1: updates are sent every 30 seconds as a broadcast
(255.255.255.255) whether or not there has been a topology
change
change
•RIPv2: updates are sent every 30 seconds as a multicast
(224.0.0.9) whether or not there has been a topology change
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Routing Table Maintenance

Periodic Updates
: distance vector protocols

Periodic

Updates
:

distance

vector

protocols

employ periodic updates to exchange routing
information with their neighbors and to maintain up
-
information

with

their

neighbors

and

to

maintain

up
-
to-date routing information in the routing table.
Failure of a link
–
Failure

of

a

link
–Introduction of a new link
–
Failure of a route
r
–Change of link parameters
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Rti TblMit
R
ou

ti
ng
T
a
bl
e
M
a
i
n
t
enance
 RIP uses 4 timers
–
U
p
date time
r
p
• interval is a route sends an update
–Invalid timer
•If an update has not been received after 180
seconds (the default) the route is marked as
seconds

(the

default)
,
the

route

is

marked

as

invalid by setting the metric to 16.
•The route is retained in the routing table until
the flush timer expires.
–
Holddown time
r
•This timer stabilizes routing information and
helps prevent routing loops during periods
when the topology is converging on new
information.
B d f lt th h ldd ti i t f 180
•
B
y
d
e
f
au
lt
,
th

e
h
o
ldd
own
ti
mer
i
s se
t

f
or
180

seconds.
–Flush timer
•By default, the flush timer is set for 240
seconds which is 60 seconds longer than the
seconds
,
which

is

60

seconds

longer

than

the

invalid timer.
•When the flush timer expires, the route is
removed from the routing table.
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Routing Table Maintenance

EIGRP
EIGRP
–Unlike other distance vector routing protocols,
EIGRP does not send periodic updates.
–
Instead
,
EIGRP sends bounded u
p
dates about a
,p
route when a path changes or the metric for that
route changes.
 EIGRP routing updates are
–Partial updates
•Updates sent only when there is a change in

topology that influences routing information
Ti db t l h
–
T
r
i
ggere
d

b
y
t
opo
l
ogy c
h
anges
–Bounded
•Propagation of partial updates are automatically
bounded so that only those routers that need the
bounded

so

that

only

those

routers

that

need

the

information are updated
–Non periodic
•
Updates are not sent out on a regular basis.
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Updates

are

not

sent

out

on

a

regular

basis.
More details on how EIGRP operates will be presented in Chapter 9.
Routing Table Maintenance
Routing

Table

Maintenance
 RIP Triggered Updates
–Routing table update that is sent immediately to adjacent
routers in response to a routing change
–
The receiving routers in turn generate triggered updates
–
The

receiving

routers
,
in

turn
,
generate

triggered

updates

that notify their neighbors of the change.

Conditions in which triggered updates are sent
Conditions

in

which

triggered

updates

are

sent
–Interface changes state
–Route becomes unreachable
–Route is placed in routing table
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Routing Table Maintenance
Routing

Table

Maintenance

RIP Triggered Updates (
problems
)
problems

RIP

Triggered

Updates

(
problems
)
–Using only triggered updates would be
sufficient if there were a guarantee that the
wave of updates would reach every
wave

of

updates

would

reach

every

appropriate router immediately.
 However, there are two problems with
triggered updates:
triggered

updates:
–Packets containing the update message can
be dropped or corrupted by some link in the
network
network
.
–The triggered updates do not happen
instantaneously. It is possible that a router that
has not
y
et received the tri
gg
ered u
p
date will
yggp
issue a regular update at just the wrong time,
causing the bad route to be reinserted in a
neighbor that had already received the
triggered update
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triggered

update
.
Triggered Extensions to RIP
Triggered

Extensions

to

RIP
Piit
Problems and Prerequisites

P
rerequ
i
s
it
es
–RIP must be enabled for this feature to
function.
function.

–This feature runs on a point-to-point,
serial interface only
–Triggered extensions to IP RIP
increase efficiency of RIP on point-to-
p

oint
,
serial interfaces.
p,
•interface serial 0
• ip rip triggered
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Routing Table Maintenance
Routing

Table

Maintenance
 Random Jitter
Synchronized updates
Synchronized

updates
A condition where multiple routers on multi access LAN
segments transmit routing updates at the same time.
Pbl
ith h i d d t

P
ro
bl
ems w

ith
sync
h
ron
i
ze
d
up
d
a
t
es
-Bandwidth consumption
-
Packet collisions (with hubs and not with switches)
Packet

collisions

(with

hubs

and

not

with

switches)

Solution to problems with
synchronized updates
- Used of random variable
called RIP_JITTER
•A good reference is : Routing TCP/IP (Jeff
Doyle) page 193-196.
•
Update timers : timer for periodic update
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Update

timers

:

timer

for

periodic

update

(default 30s) - RIP_JITTER (random to
prevent colision - 15% of the update timers)
Routing Table Maintenance
Routing

Table

Maintenance
 Random Jitter
•
Figure 5 1 RIP adds a small random variable to the update timer
•
Figure

5
.
1
.
RIP

adds

a

small

random

variable

to

the

update

timer

at each reset to help avoid routing table synchronization. The
RIP updates from Cisco routers vary from 25.5 to 30 seconds, as
h i th d lt ti f th d t
s
h
own
i
n
th
e
d
e
lt
a
ti
mes o
f

th
ese up
d
a
t
es.
Routing TCP/IP,
Volume I (CCIE

Professional
Professional

Development)
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/>velopment/source/1578700418/ch05lev1sec1.html#ch05fig1
Rti L
R
ou
ti
ng
L
oops
 Routing loops are
A

co
n
d
i
t
i
o
n in whi
c
h
a

A

condition

in

which

a

packet is continuously
transmitted within a
ift
ser
i
es o
f
rou
t
ers
without ever reaching
its destination.
its

destination.
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Routing Loops
 Routing loops may be caused by:
-Incorrectly configured static routes
-Incorrectly configured route redistribution
-Slow convergence
Incorrectl config red discard ro tes
-
Incorrectl
y
config
u
red

discard

ro
u
tes
 Routing loops can create the following issues
Excess use of bandwidth
-
Excess

use

of

bandwidth
-CPU resources may be strained
-

Network convergence is degraded
Network

convergence

is

degraded
-Routing updates may be lost or not processed in a timely
manner
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