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© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
ITE PC v4.0
Chapter 1 1
OSPF
Capítulo 11

ITE PC v4.0
Chapter 1 2
Introduction

ITE PC v4.0
Chapter 1 3
Introduction to OSPF
OSPF Packet Types

ITE PC v4.0
Chapter 1 4
 OSPF Hello Intervals
–Usually multicast (224.0.0.5)
–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

ITE PC v4.0
Chapter 1 5
OSPF Algorithm
 OSPF routers build &
maintain link-state
database containing LSA
received from other routers
–Information found in database
is utilized upon execution of
Dijkstra SPF algorithm
–SPF algorithm used to create
SPF tree
–SPF tree used to populate
routing table

ITE PC v4.0
Chapter 1 6
Basic OSPF Configuration
The router ospf command
 To enable OSPF on a router use the following
command
R1(config)#router ospf process-id
Process id
 A locally significant number between 1 and 65535
-this means it does not have to match other OSPF
routers

ITE PC v4.0
Chapter 1 7
 OSPF network command
-Requires entering: network address
wildcard mask - the inverse of the subnet
mask
area-id - area-id refers to the OSPF area.
OSPF area is a group of routers
that share link state information
-Example: Router(config-router)#network network-address
wildcard-ask area area-id

ITE PC v4.0
Chapter 1 8
 Router ID
–This is an IP address used to identify a router
–3 criteria for deriving the router ID
Use IP address configured with OSPF router-id command
-Takes precedence over loopback and physical interface
addresses
If router-id command not used then router chooses highest
IP address of any loopback interfaces
If no loopback interfaces are configured then the highest IP
address on any active interface is used
–Show ip protocols
–Show ip ospf
–Show ip ospf interface

ITE PC v4.0
Chapter 1 9
Verifying OSPF
 Use the show ip ospf neighbor command to verify &
troubleshoot OSPF networks
Command will display the following:
 Neighbor adjacency
-No adjacency indicated by -
Neighboring router’s Router ID is not displayed
A state of full is not displayed
-Consequence of no adjacency-
No link state information exchanged
Inaccurate SPF trees & routing tables

ITE PC v4.0
Chapter 1 10
OSPF Metric
 OSPF uses cost as the metric for determining the
best route
-The best route will have the lowest cost
-Cost is based on bandwidth of an interface
Cost is calculated using the formula
108
/ bandwidth
-Reference bandwidth
defaults to 100Mbps
can be modified using
auto-cost reference-bandwidth command

ITE PC v4.0
Chapter 1 11
Modifying the Cost of a link
 Both sides of a serial link should be configured with the
same bandwidth
–Commands used to modify bandwidth value
Bandwidth command
–Example: Router(config-if)#bandwidth bandwidth-kbps
ip ospf cost command – allows you to directly specify
interface cost
-Example:R1(config)#interface serial 0/0/0
R1(config-if)#ip ospf cost 1562

ITE PC v4.0
Chapter 1 12
OSPF in Multiaccess Networks
 2 challenges presented by multiaccess networks
–Multiple adjacencies
–Extensive LSA flooding

ITE PC v4.0
Chapter 1 13
 Solution to LSA flooding issue
is the use of
–Designated router (DR)
–Backup designated router (BDR)
 DR & BDR selection
–Routers are elected to send &
receive LSA
 Sending & Receiving LSA
–DRothers send LSAs via multicast
224.0.0.6 to DR & BDR
–DR forward LSA via multicast
address 224.0.0.5 to all other
routers

ITE PC v4.0
Chapter 1 14
DR/BDR Election Process
 DR/BDR elections DO NOT occur in point to point
networks

ITE PC v4.0
Chapter 1 15
 Criteria for getting elected DR/BDR
1. DR: Router with the highest OSPF
interface priority.
2. BDR: Router with the second highest
OSPF interface priority.
3. If OSPF interface priorities are equal, the
highest router ID is used to break the tie.

ITE PC v4.0
Chapter 1 16
Hello Protocol
 OSPF Hello Packet
–Discover OSPF neighbors & establish adjacencies
–Advertise guidelines on which routers must agree to become neighbors
–Used by multi-access networks to elect a designated router and a
backup designated router

ITE PC v4.0
Chapter 1 17
 Timing of DR/BDR Election
–Occurs as soon as 1st
router has its interface enabled on
multiaccess network
When a DR is elected it remains as the DR until one of the
following occurs
-The DR fails.
-The OSPF process on the DR fails.
-The multiaccess interface on the DR fails.

ITE PC v4.0
Chapter 1 18
OSPF Interface Priority
 Manipulating the DR/BDR election process continued
–Use the ip ospf priority interface command.
–Example:Router(config-if)#ip ospf priority {0 - 255}
Priority number range 0 to 255
–0 means the router cannot become the DR or BDR
–1 is the default priority value

ITE PC v4.0
Chapter 1 19
More OSPF Configuration
Redistributing an OSPF Default Route
 Topology includes a link to ISP
–Router connected to ISP
Called an autonomous system border router
Used to propagate a default route
–Example of static default route
R1(config)#ip route 0.0.0.0 0.0.0.0
loopback 1
R1(config-router)#default-information
originate

ITE PC v4.0
Chapter 1 20
More OSPF Configuration
Fine-Tuning OSPF
 Modifying OSPF timers
–Reason to modify timers
Faster detection of network failures
–Manually modifying Hello & Dead intervals
Router(config-if)#ip ospf hello-interval seconds
Router(config-if)#ip ospf dead-interval seconds
–Point to be made
Hello & Dead intervals must be the same between
neighbors

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