Routing Protocols and Concepts – Chapter 11 potx

OSPF (Single Area OSPF)
Routing Protocols and Concepts – Chapter 11
© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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Introduction
•In this chapter, you will learn basic, single-area OSPF implementations
and configurations
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and

configurations
.
•More complex OSPF configurations and concepts (multi-areas OSPF)
are reserved for CCNP-level courses.
Introduction to OSPF
Introduction

to

OSPF
Background of OSPF
 Began in 1987
 1989 OSPFv1 released in RFC 1131
This version was experimental & never deployed
 1991 OSPFv2 released in RFC 1247
 1998 OSPFv2 updated in RFC 2328

 1999 OSPFv3
p
ublished in RFC 2740
p
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Introduction to OSPF
Introduction

to

OSPF
OSPF Message Encapsulation

OSPF packet type
OSPF

packet

type
– There exist 5 types (next slide)
 OSPF packet header
–Contains - Router ID an area ID
and Type code for OSPF packet
type
 IP packet header
– Contains - Source IP address,
D

es
tin
a
ti
o
n IP
add
r
ess,

&
Pr
o
t
oco
l
es a o add ess, & o oco
field set to 89. the destination
address is set to one of two
multicast addresses: 224.0.0.5 or
224006
224
.
0
.
0
.
6
.
 Data Link Frame Header

–
Contains - destination MAC address is
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also a multicast address: 01-00-5E-00-
00-05 or 01-00-5E-00-00-06.
I t d ti t OSPF
I
n
t
ro
d
uc
ti
on
t
o
OSPF
5 OSPF Packet Types:
 1. Hello - Hello packets are used to establish and
maintain adjacency with other OSPF routers.
 2. DBD - The Database Description (DBD) packet
contains an
abbre iated list of the sending ro ter's
contains

an

abbre
v
iated

list

of

the

sending

ro
u
ter's

link-state database and is used by receiving
routers to check against the local link-state
database.
 3. LSR - Receiving routers can then request more
information about any entry in the DBD by sending
a Link-State Request (LSR).
 4. LSU - Link-State Update (LSU) packets are
used to reply to LSRs as well as to announce new
information.
–
LSUs contain 7 different types of Link-State
Advertisements (LSAs).
–LSUs and LSAs are discussed in a later topic.
5LSAk

Wh LSU i i d th t
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
5
.
LSA
c
k
-
Wh
en an
LSU

i
s rece
i
ve
d
,
th
e rou
t
er
sends a Link-State Acknowledgement (LSAck) to
confirm receipt of the LSU.
OSPF: Hello Protocol
OSPF:

Hello

Protocol
 Purpose of Hello Packet

Discover OSPF neighbors & establish adjacencies

Discover

OSPF

neighbors

&

establish

adjacencies
 Advertise parameters on which routers must agree to become
neighbors
 Used by multi-access networks to elect a Designated Router and
a Backup Designated Router
 Type: OSPF Packet Type: Hello (1), DD (2), LS
Request (3), LS Update (4), LS ACK (5)
 Router ID: ID of the originating router
 Area ID: area from which the packet originated
 Network Mask: Subnet mask associated with the
sending interface
sending

interface
 Hello Interval: number of seconds between the
sending router's hellos
 Router Priority: Used in DR/BDR election (discussed
lt )
l
a
t
er
)
 Designated Router (DR): Router ID of the DR, if any
 Backup Designated Router (BDR): Router ID of the
BDR, if any
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 List of Neighbors: lists the OSPF Router ID of the
neighboring router(s)
OSPF: Hello Protocol
OSPF:

Hello

Protocol
 Establish adjacencies:
–
The
y

must a
g
ree on three values: Hello
Why 10 second hello interval
Also need to have the
same Area ID.
yg
interval, Dead interval, and network type.
 OSPF Hello Intervals
–Hello interval indicates how often an OSPF
router transmits its Hello packets
Why

10

second

hello

interval

communications consider better than
the 30 second routing update for RIP?
router

transmits

its

Hello

packets
–Usually multicast (224.0.0.5) for
ALLSPFRouters
–
sent ever
y
10 seconds on multiaccess and
y
point-to-point segments
–Sent every 30 seconds for NBMA segments
 OSPF Dead Intervals
–
This is the time that must transpire before the
neighbor is considered down
–Default time is 4 times the hello interval
–
For multiaccess and point
-
to
-
point segments
For

multiaccess

and

point
to

point

segments
,
this period is 40 seconds.
–For NBMA networks, the Dead interval is 120
seconds.
If the Dead interval expires before the routers
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–
If

the

Dead

interval

expires

before

the

routers

receive a Hello packet, OSPF will remove that

neighbor from its link-state database.
OSPF: Hello Protocol

To reduce the amount of OSPF traffic on
To

reduce

the

amount

of

OSPF

traffic

on

multiaccess networks, OSPF elects a
Designated Router (DR) and Backup
Desi
g
nated Router
(
BDR
)
.
g()

 Hello protocol packets contain information that
is used in electing DR and BDR
The DR is responsible for updating all other
–
The

DR

is

responsible

for

updating

all

other

OSPF routers (called DROthers) when a change
occurs in the multiaccess network.
–
The BDR monitors the DR and takes over as
The

BDR

monitors

the

DR

and

takes

over

as

DR if the current DR fails.
 In the figure, R1, R2, and R3 are connected
through point
to
point links Therefore no
More detail discussion
through

point
-
to
-
point

links
.
Therefore
,

no

DR/BDR election occurs.
–The DR/BDR election and processes will be
discussed in a later topic and the topology will
More

detail

discussion

on the DR, BDR,
DROther later. You
need to know this for
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discussed

in

a

later

topic

and

the

topology

will

be changed to a multiaccess network.
need

to

know

this

for

CCNA exam.
Introduction to OSPF
Introduction

to

OSPF
OSPF Link-state Updates

Purpose of a Link State Update (LSU)
Purpose

of

a

Link

State

Update

(LSU)
–Used to deliver link state advertisements
 Purpose of a Link State Advertisement (LSA)
–Contains information about neighbors & path costs
–An LSU packet can contain 11 different types of LSAs,
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Introduction to OSPF
Introduction

to

OSPF
OSPF Algorithm
 OSPF routers build &
maintain link-state
dtb
tii LSA
d

a
t
a
b
ase con
t
a
i
n
i
ng
LSA

received from other
routers
routers
1. Information found in
database is utilized upon
e ec tion of Dijkstra SPF
e
x
ec
u
tion

of

Dijkstra

SPF

algorithm
2. SPF al
g
orithm used to
g
create SPF tree
3. SPF tree used to populate
routing table
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routing

table
Introduction to OSPF
Introduction

to

OSPF
Administrative Distance
Administrative

Distance
 Default Administrative Distance for OSPF is 110
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Introduction to OSPF
Introduction

to

OSPF
 OSPF Authentication
–
It is good practice to authenticate transmitted
It

is

good

practice

to

authenticate

transmitted

routing information.
–
This is an interface s
p
ecific confi
g

uration
pg
–This practice ensures that routers will only accept
routing information from other routers that have been
configured with the same password or authentication
configured

with

the

same

password

or

authentication

information
MD5 authentication
kIDtht
Note: Authentication
does not encrypt the
router
'
s routing table
uses a
k
ey

ID

th
a
t

allows the router to
reference multiple
p
asswords
,
makin
g

?
router s

routing

table
.
p,g
password migration
easier and more
secure.
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?

Basic OSPF Configuration
Lab Topology
Lab

Topology
 Topology used for this chapter
–Discontiguous IP addressing
scheme
–Since OSPF is a classless
routing protocol the subnet mask
is will be configured as part of our
is

will

be

configured

as

part

of

our

OSPF configuration.
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Basic OSPF Configuration
Basic

OSPF

Configuration
The router ospf command
The

router

ospf

command
 To enable OSPF on a router use the following
command
R1(config)#router ospf process-id
Process id
ID cannot be 0
 A locally significant number between 1 and 65535
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Basic OSPF Configuration
Basic

OSPF

Configuration
 OSPF network command
–
Re
q
uires enterin
g
:
qg
•network address
•wildcard mask - the inverse of the subnet mask
•
area
-
id
-
area
-
id refers to the OSPF area OSPF area
•
area
-
id
-
area
-
id

refers

to

the

OSPF

area
.
OSPF

area

is a group of routers that share link state information
Router(config
-
router)#
network
network
-
address wildcard
-
ask
area
area
-
id
Router(config
router)#
network

network
address

wildcard
ask

area
area
id
255.255.255.255
- 255.255.255.240

00 015
Subtract the
subnet mask
Wildcard mask
0
.
0
.
0
.
15
255.255.255.255
- 255.255.255.252
Subtract the
subnet mask
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0. 0. 0. 03
Wildcard mask
Basic OSPF Configuration
Basic

OSPF

Configuration
 Cisco IOS now properly handles overlapping network area configuration commands.
 Consider the followin
g
exam
p
le:
gp
fw#conf t
Enter configuration commands, one per line. End with CNTL/Z.
fw(config)#router ospf 100
fw
(
config
router)#
network 0 0 0 0 255 255 255 255 area 0
fw
(
config
-
router)#

network

0
.
0
.
0
.
0

255
.
255
.
255
.
255

area

0
fw(config-router)#network 10.0.0.0 0.0.3.255 area 1
13:06:57: %OSPF-6-AREACHG: 10.0.0.0 255.255.252.0 changed from area 0 to area 1
fw(config-router)#network 10.0.0.0 0.0.0.7 area 2
13:07:10: %OSPF-6-AREACHG: 10.0.0.0 255.255.255.248 changed from area 1 to area 2
fw(config-router)#^Z
 I've entered overlapping network statements, each one with a smaller address range. Not
l d IOS d t t th t th l it l i t i
l
dd

on
l
y
d
oes
IOS

d
e
t
ec
t

th
a
t

th
ey over
l
ap,
it
a
l
so pr
i
n
t
s n
i

ce sys
l
og messages an
d
reor
d
ers
the commands in the running configuration. Well done !
fw#show run | begin router ospf
router os
p
f 100
p
log-adjacency-changes
network 10.0.0.0 0.0.0.7 area 2
network 10.0.0.0 0.0.3.255 area 1
network0000255255255255area0
o/2006/11/
network-statements-in-ospf-
ht l
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network

0
.
0
.

0
.
0

255
.
255
.
255
.
255

area

0

process-are.
ht
m
l
Basic OSPF Configuration
Basic

OSPF

Configuration
 ospf network definition for adding all interfaces / default route
What’s the difference?
router ospf 1
network 0.0.0.0 0.0.0.0 area 0

vs
vs
.
router ospf 1
network 0.0.0.0 255.255.255.255 area 0
 Both add all existing interfaces into area 0 and all later added interfaces
also. Both statements are valid.
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B i OSPF C fi ti
B
as
i
c
OSPF

C
on
fi
gura
ti
on

Area area
id

Area

area
-
id
An OSPF area is a group of routers that share link-state
information.
 In this chapter, we will configure all of the OSPF routers within a
single area. This is known as single-area OSPF.
Multi-area OSPF is covered in CCNP.
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B i OSPF C fi ti
B
as
i
c
OSPF

C
on
fi
gura
ti
on
 Router ID
– This is an IP address used to identify a router
– 3 criteria for deriving the router ID
1 U IP dd fi d ith OSPF
t

id
d
1
.
U
se
IP
a
dd
ress con
fi
gure
d
w
ith

OSPF
rou
t
er
-
id
comman
d
-Takes precedence over loopback and physical interface
addresses
2. If router-id command not used then router chooses highest
IP address of any loopback interfaces
3
If no loopback interfaces are configured then the

highest IP
3
.
If

no

loopback

interfaces

are

configured

then

the

highest

IP

address on any active physical interface is used
 The interface does not need to be enabled for OSPF,
meaning that it does not need to be included in one of the
meaning

that

it

does

not

need

to

be

included

in

one

of

the

OSPF network commands.
 However, the interface must be active - it must be in the
up state
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up

state
.
However!!!!!!
B i OSPF C fi ti
B
as
i
c
OSPF

C
on
fi
gura
ti
on
RtID

R
ou
t
er
ID
If th ki h th
–
If
you are
th
e

ki
ng w
h
en
th
e
kingdom is built, you are the
kingdom

is

built,

you

are

the

KING for life
– It means when ID is elected, it
ithIDf th t l
i
s
th
e
ID

f
or

th
e rou
t
er, un
l
ess
…………
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Basic OSPF Configuration
Basic

OSPF

Configuration
OSPF Router ID
 Commands used to verify current router ID
–Show ip protocols
–Show ip ospf
–Show ip ospf interface
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Basic OSPF Configuration
Basic

OSPF

Configuration
OSPF Router ID
 Router ID (not configured) & Loopback addresses
(configured)
–
Highest loopback address will be used as router ID
Highest

loopback

address

will

be

used

as

router

ID
–Advantage of using loopback address the loopback
interface cannot fail Æ OSPF stability

The OSPF router
-
id command

The

OSPF

router
id

command
–Introduced in IOS 12.0
–OSPF router-id command, which is a fairly recent
addition to IOS
,
it is more common to find loo
p
back
,p
addresses used for configuring OSPF router IDs.
–Command syntax
Router(config)#router ospf process-id
Router(config-router)#router-id ip-address
 Modifying the Router ID
–
Use the command
Router
#clear ip
ospf process
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Use

the

command
Router
#clear

ip
ospf

process
This command does not work in PT.
Basic OSPF Configuration
Basic

OSPF

Configuration
Modif
y
in
g
the Router ID
yg
 The router ID is selected when OSPF is
configured with its first OSPF network command.
–
If the OSPF router
-

id command or the loopback
If

the

OSPF

router
id

command

or

the

loopback

address is configured after the OSPF network
command, the router ID will be derived from the
interface with the highest active IP address.
f
 Modi
f
ying the Router ID
The router ID can be modified with
1. the IP address from a subsequent OSPF router-id
command b reloading the ro ter or
command

b
y
reloading

the

ro
u
ter

or

2. by using the following command:
Router#clear ip ospf process
3. Modifying a router ID with a new loopback or
physical interface IP address may require
reloading the router
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reloading

the

router
Basic OSPF Configuration
Basic

OSPF

Configuration
Duplicate Router IDs
Duplicate

Router

IDs
 When two routers have the same router ID in
an OSPF domain, routing may not function
properly.
properly.

– If the router ID is the same on two
neighboring routers, the neighbor
establishment may not occur.
 When duplicate OSPF router IDs occur, IOS
will display a message similar to:
– %OSPF-4-DUP_RTRID1: Detected router
ith d li t t ID
w
ith

d
up
li
ca
t
e rou
t

er
ID
 To correct this problem, configure all routers
so that they have unique OSPF router IDs.
¾ Because some IOS versions do not support
the router-id command, we will use the
loopback address method for assigning
router IDs
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router

IDs
.
Quick Review
Quick

Review
We just went over 3 different types of ID
We

just

went

over

3

different

types

of

ID
 ospf process-id.
OSPF process
–
OSPF

process
.
– Cannot be 0

Area ID:

Area

ID:
– OFPS area
–
If it is the first
,
and the backbone area
,
it is 0
,,

 Router ID
–
Router ID
– 1 IP address is elected per router,
• Highest physical address (or)
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• Highest logical address (loopback)